Petrography, depositional environments, and diagenesis ofBisbee Group carbonates, Guadalupe Canyon area, Arizona

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PETROGRAPHY, DEPOSITIONAL ENVIRONMENTS, AND DIAGENESIS

OF BISBEE GROUP CARBONATES, GUADALUPE CANYON AREA, ARIZONA

by

Robert C lark Ferguson

A Thesis Submitted to the Facu lty o f the

DEPARTMENT OF GEOSCIENCES

In P a r t ia l F u lf il lm e n t o f the Requirements For the Degree o f

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

1 9 8 3

OC <u

STATEMENT BY AUTHOR

This th e s is has been submitted in p a r t ia l f u l f i l l m e n t o f re ­quirements fo r an advanced degree a t the U n iv e rs i ty o f Arizona and is deposited in the U n iv e rs i ty L ib ra ry to be made a v a i la b le to borrowers under ru les o f the L ib ra ry .

B r ie f quota tions from th is th e s is are a llow ab le w ith o u t special perm ission, provided th a t accurate acknowledgment o f source is made. Requests fo r permission fo r extended quo ta tion from or reproduction o f th is manuscript in whole o r in p a r t may be granted by the head o f the major department or the Dean o f the Graduate College when in h is judg ­ment the proposed use o f the m a te r ia l is in the in te re s ts o f sch o la r­sh ip . In a l l o ther ins tances, however, permission must be obtained from the author.

SIGNED:

APPROVED BY THESIS DIRECTOR

This th e s is has been approved on the date shown below:

Professor o f Geosciences

ACKNOWLEDGEMENTS

I wish to thank Dr. Joseph F. Schreiber, J r . fo r suggesting th is

research top ic and fo r his many valuab le comments in the f ie ld and in

the lab o ra to ry . I am g ra te fu l to Dr. Karl W. Flessa who also made

h elp fu l comments in the f ie ld . Drs. W illiam R. Dickinson* Karl W.

Flessa and Joseph F. S chreiber, J r . c r i t i c a l l y reviewed th is m anuscript.

F inancial support fo r th is research was generously supplied by

the ARCO Exploration Company, the Laboratory o f G eotectonics, U n iv e rs ity

of A rizona, and a Bert S. B u tle r scholarship .

I wish to extend a special thanks to the John Magoffin fa m ily ,

ranchers in the Guadalupe Canyon a rea , fo r a llow ing access to th e ir land

and fo r th e ir warm h o s p ita lity .

TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS........................................................................... v

ABSTRACT....................................................................................................... ............... v i i i

INTRODUCTION............................................................................................... 1

PREVIOUS WORK.............................................................................................................. 5

REGIONAL TECTONIC SETTING AND CORRELATIONS.................................................... 8

LOWER CRETACEOUS OF THE GUADALUPE CANYON A R E A ... . . ................................. 16

Geologic S e tt in g .................................................................................................. 16S tra t ig ra p h y ....................................................................................... 18S tru c tu re .......................................................................... 20

PETROGRAPHY, DEPOSITIONAL ENVIRONMENTS, AND DIAGENESIS............... 25

In tro d u c tio n ........................................................................................................... 25Mixed C la s tic and Carbonate S ection , M orita Form ation .. . . 26Massive Carbonates, Lower Mural Limestone.......................................... 47Non-Reefal Upper Mural Limestone.............................................................. 54

PETROLEUM POTENTIAL....................................................................................................... 60

SUMMARY.......................................................................................................................... 63

APPENDIX........................................................................................................................ 65

REFERENCES.......................................................................................................................... 88

iv

LIST OF ILLUSTRATIONS

Figure Page

1. S tra tig ra p h ic column-type sec tio n , Bisbee Group......... 2

2. Location map o f study a re a ........................................................ 3

3. E arly Cretaceous paleogeographic reconstructionsof southeastern A rizona............................................................... 6

4. Paleotecton ic map o f the southern C o rd ille ra (m id-Late Jurassic - la te s t C retaceous)...................................... 9

5. Conceptual models o f la te Mesozoic southernC o rd ille ra backarc te c to n ic s .................................................... 10

6. C o rre la tio n chart - Cretaceous of southeasternArizona and southwestern New Mexico.................................... 12

7. Reconnaissance geologic map (1 :1 25 ,000 ) o f studyarea ......................................................................................................... 17

8. Composite section - Mural Limestone,Guadalupe Canyon a rea .................................................................... 19

9. Fence diagram - upper Mural Limestone,extreme southeastern A rizona.................................................... 21

10. Map o f prominent Lower Cretaceous carbonate outcrops in study area - constructed as an overlay on a1:65,000 scale a e r ia l photograph........................................... 22

11. Large, plunging syn c lin e , Lower Cretaceous,Guadalupe Canyon a re a .................................................................... 24

12. Codiacean a lg a l fragment - Halimeda?.................................. 27

13. Overgrowths on terrigenous quartz g ra in ........................... 27

14. M o ttled , do lom itic lim estone.................................................... 29

15. P a r t ia l ly dolom itized p e lm ic rite - pel s p a n 'te .............. 29

16. Is o la te d , h em atite -s ta in ed , euhedral, dolom iterhombohedron........................................................................................ 30

v

vi

LIST OF ILLUSTRATIONS— Continued

Page

17. Iron-zoned dolom ite rhombohedron.. . . . . . . . . . . . . . . . . . . 30

18. M ic r ite rims in a fo s s ilife ro u s o o s p arite ........................ 32

19. Authigenic quartz a f te r .an h yd rite .......................................... 32

20. Authi genic quartz a f te r an h yd rite .......................................... 33

21. Pseudofibrous e x tin c tio n in an auth igen icquartz c rys ta l a f te r an h yd rite ............................................... 33

22. Vuggy p o ro s ity in a mi c r i t i c lim estone............................. 35

23. Pressure so lu tio n in g in a do lom itic o o s p arite .............. 35

24. S ty le l ite s in a fo s s ilife ro u s o o s p a r ite -o o m ic r ite .. . 36

25. Id e a lize d carbonate-evaporite sequence (shallowsubtidal to supra tida l c o n d itio n s )...................................... 38

26. Scour surface , and in tra c la s ts from a d o lo m itic ,fo s s il ife rou s o o s p arite ............................................................... 40

27. P a r t ia l ly s i l i c i f i e d b u r ro w -f i l l sediment....................... 40

28. S tro m a to lite s ..................................................................................... 42

29. S i l ic i f ic a t io n o f m ic r ite adjacent to c a lc if ie dworm tubes............................................................................................ 46

30. S i l ic i f i e d o o lite s in a fo s s ilife ro u s o o s p arite ......... 46

31. O n c o lit ic lim estone........................................................................ 48

32. G irvan e lla tu b u les .......................................................................... 48

33. Botryoidal lumps in a fo s s ilife ro u s in t r a m ic r i t e . . . . 50

34. Massive, ceroid corals (A c tin as trea? ) andbranching, cero id corals in m ic r ite -r ic h sed im en t... 50

35. Overturned, hem ispheroidal, ceroid coral(A c tin as trea? ) ................................................................................... 51

36. Growth lin e s in branching, phaceloidcoral (C a lam ophyllia? )................................................................. 51

v u

LIST OF ILLUSTRATIONS— Continued

Page

37. Sparry c a lc i t e - f iH e d in tra s k e le ta l p o ro s ity in anO rb ito lin a t e s t ......................... ....................................................... 56

38. Sparry c a lc i t e - f i l i e d in te rp e lle ta l p o ro s ity ................ 56

39. Chert nodules in mi c r i t i c sediment...................................... 59

ABSTRACT

Lower Cretaceous outcrops in the Guadalupe Canyon area consist

of the M orita Formation and Mural Limestone. A mixed c la s t ic and

carbonate section in the M orita Formation marks the f i r s t Lower

Cretaceous carbonate deposition in the study area. Shallow subtidal to

supratidal sediments were deposited in an a rid environment. Period ic

m eteoric water in f lu x induced local sch izohaline conditions and

associated diagenesis o f the carbonate sediments.

While the upper Mural is equ iva len t in thickness to the type

sec tio n , the lower Mural is tw ice as th ic k . The two members are in te r -

bedded in the study a rea , w ith a th in s tr in g e r o f upper Mural located

w ith in the lower M ural. This suggests th a t the two in te r f in g e r over

great d istances. The upper Mural represents the major period o f Lower

Cretaceous carbonate deposition in the area. M ic r i t ic sediments were

deposited in a g e n era lly low energy, s l ig h t ly reducing, s h e lf environ­

ment w ith scattered c o r a l-a lg a l- r u d is t patch reefs and associated

shoals.

v iii

INTRODUCTION

The Bisbee Group, o f E arly Cretaceous age, was f i r s t named by

Ransome (1904) from exposures in the Mule Mountains near Bisbee,

Arizona. Ransome subdivided the Bisbee Group in to four formations: the

basal Glance Conglomerate, the M orita Formation, the Mural Limestone,

and the Cintura Formation (Figure 1 ). These formations crop out in

mountain blocks over much o f southeastern Arizona, where they are e a s ily

recognized as a c lass ic transgressive-regressive sequence. S ituated be­

tween the fe ld sp ath ic sandstones, s iIts to n e s , and mudstones of the

M orita and Cintura Formations, the Mural Limestone represents the

maximum extent o f the Early Cretaceous transgression in to southeastern

Arizona. Ransome (1904) divided the Mural in to two members. The upper

member is dominated by th ic k ly bedded, r e la t iv e ly pure, fo s s ilife ro u s

limestones th a t are topographica lly expressed as re s is ta n t c l i f f s and

ridges, w hile the lower member consists of interbedded calcareous sand­

stones, s ilts to n e s , mudstones, and impure fo s s ilife ro u s limestones th a t

are topographica lly expressed as ledges and slopes.

This study concentrates on the carbonates w ith in the M orita

Formation and Mural Limestone in the Guadalupe Canyon area o f extreme

southeastern Arizona and southwestern New Mexico (F igure 2 ) . This area

has previously received l i t t l e d e ta ile d study, and a f te r in i t i a l recon­

naissance f ie ld work, was found to contain many e xc e llen t Lower Cre­

taceous outcrops. The study o f these carbonate rock outcrops has yie lded

1

GtANCECGI.

Figure 1: S tra tig ra p h ic column-type sectionBisbee Group ( a f t e r S c o tt, 1 9 7 9 ).

Figure 2. Location map o f study area.

Selected previous Lower Cretaceous s tu d ies :

1. J . L . Ransome, Lower Cretaceous2. A. Stoyanow, Lower Cretaceous3. S. B. S in d lin g e r , lower Mural Limestone4. G. H. Roybal, upper Mural Limestone5. G. R. Grocock, upper Mural Limestone6. L . A. L indberg , Lower Cretaceous

Key

- study area lo c a tio n

M E X I C O

Figure 2.

NE

W

ME

XIC

O

4

inform ation on the nature o f the E arly Cretaceous transgression in to

southeastern Arizona and i ts spectra o f depositional environments. In

add iton , w ith the Guadalupe Canyon area being s itu a te d in a basinward

pos ition w ith respect to p rev ious ly studied Lower Cretaceous outcrops in

Arizona, the depositional environments ana lys is supplements regional

l ith o fa c ie s models o f southeastern Arizona during the E arly Cretaceous.

Observation o f d iagenetic features has y ie lded inform ation on post-

depositional d iagenetic environments.

F ie ld work included measuring and sampling s ix s tra tig ra p h ic

sections (appendix A ), and the reconnaissance geologic mapping o f the

Lower Cretaceous in the Guadalupe Canyon area . Laboratory studies

included the petrographic examination o f n in e ty three th in sections, as

w ell as the preparation o f carbonate slabs and aceta te p ee ls , inso lub le

residue a n a ly s is , x -ra y d if f r a c t io n work, and SEM stud ies .

PREVIOUS WORK

Since Ransome f i r s t named and described the Bisbee Group,

several U .S.G .S. Professional Papers have been published in which Bisbee

Group formations are b r ie f ly described as p a rt o f a general review o f

the geology and Mesozoic s tra tig ra p h y o f various mountain ranges in

southeastern Arizona ( G i l lu ly , 1956; Hayes, 1970a). Hayes (1970b) made

the most s ig n if ic a n t c o n trib u tio n when he summarized the e x is tin g

knowledge of the s tra tig ra p h y of the Bisbee Group in a study o f the

Cretaceous paleogeography o f southeastern Arizona (F igure 3 ) .

More re c e n tly , a tte n tio n has focused on the c o ra l-a lg a l- ru d is t

patch reefs o f the upper member of the Mural Limestone (S c o tt, 1974,

1979, 1981; Grocock, 1975; Roybal, 1979), and th e ir importance as an

environmental analogue to the productive S tu art C ity Formation (Lower

Cretaceous) o f the G ulf Coast reg ion. L i t t l e d e ta ile d work has been

done on the carbonates o f the lower member o f the Mural Limestone and

the M orita Formation except fo r several pa leon to log ica l s tud ies .

Stoyanow's (1949) d e ta ile d paleon to log ical studies in the N inety One

H il ls area led to his renaming o f Ransome's o r ig in a l Mural u n it . The

name Mural Limestone was re s tr ic te d to Ransome's upper member w h ile the

lower member was renamed the Lowell Formation. Subsequent work in the

area (Hayes and Landis, 1961) showed Stoyanow to have incorporated the

upper beds o f the M orita Formation in to his Lowell Formation and found

the o r ig in a l nomenclature o f Ransome a more s u ita b le mapping base.

5

Fig

ure

3. E

arly Cretaceous

paleogeographic

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7

S. S in d lin g er (1981) completed a p a leo n to lo g ic a l-o rie n te d study on fo u r

lower Mural sections in several southeastern Arizona mountain ranges,

and F.A. Lindberg (1982) completed a sed im ento log ica l-o rien ted study o f

the Cretaceous section exposed in the Rucker Canyon horst block o f the

southern Chiricahua mountains. The Guadalupe Canyon area l ie s to the

southeast o f these lo c a le s , and except fo r the two upper Mural Limestone

studies completed there (Grocock, 1975; S c o tt, 1979), the Lower

Cretaceous section has remained untouched.

REGIONAL TECTONIC SETTING AND CORRELATIONS

Bisbee Group sediments were deposited in the northwestern end o f

the Chihuahua Trough, an extensional basin of Late Jurassic to mid-

Cretaceous age, connected to the G ulf o f Mexico (F igure 4 ) . The marine

transgression recorded in the lower formations o f the Bisbee Group was a

re s u lt o f the therm otectonic subsidence o f the G u lf o f Mexico and i t s

margins a f te r mid-Mesozoic r i f t in g (D ickinson, 1981). The sea advanced

northward up the Chihuahua Trough from the G u lf, reaching i ts maximum

exten t during the deposition o f the upper Mural Limestone in E arly

Albian tim e. Bilodeau (1979) proposed three possib le conceptual models

to exp la in the backarc tecton ics o f the southern C o rd ille ra during the

Cretaceous (F igure 5 ) . The three include: an aulacogen; v a r ia b i l i t y in

absolute motion o f the o verrid in g North American lith o s p h e ric p la te ;

and v a r ia b i l i t y in the descent dip o f the subducting Fara lIon

lith o s p h e ric p la te . The la te r two models neg lect tim in g , as the Rocky

Mountain re tro a rc fore land basin is o f Late Cretaceous age, and thus

younger than the Chihuahua Trough.

Lower Cretaceous sediments deposited in the northern portio n o f

the Chihuahua Trough extend today from outcrops in southeastern Arizona

and southwestern New Mexico southeastward to Trans-Pecos Texas and

northern Chihuahua, Mexico. Z e l le r (1965) described a Lower Cretaceous

section in the Big Hatchet Mountains o f New Mexico which c lo s e ly re ­

sembles the Bisbee Group s tra tig ra p h y . The section consists o f the

8

CD HI CO

LU O

'

Figure 4 . Paleotectonic map o f the southern C o rd ille ra (m id-Late J u ra s s ic -la te s t Cretaceous) (a f te r D ickinson, 1981). L e tte rs designate the lo catio n o f the study area and Mural Limestone time - equ iva len t form ations.

__________________________ Key_______________________________

Mural Limestone, Guadalupe Canyon area U-Bar Formation, Big Hatchet Mountains B lu ff Mesa Limestone, S ie rra Blanca area B lu ff Formation, Eagle Mountains Quitman Formation, Quitman Mountains

Figure 4.

Figure 5. Three possible conceptual models of la te Mesozoic southern C o rd ille ra backarc tecton ics (a f te r B ilodeau, 1979). The three include: A) an aulacogen, B) v a r ia b i l i t y in the descent dip o f the subducting Fara llon lith o s p h e ric p la te , and C) v a r i ­a b i l i t y in the absolute motion o f the overrid ing North American lith o s p h e ric p la te .

§ 1 / '

Figure 5.

Hell to Fin ish Formation, the U-Bar Formation, and the Mojado Formation.

The H ell to F in ish Formation is dominated by mudstones, sha les , s i l t -

stones, and sandstones w ith a basal conglomerate res tin g unconformably

on Paleozoic carbonates. The overly ing U-Bar Formation consists m ainly

of interbedded e la s tic s and carbonates w ith a massive, biohermal lim e­

stone, r ic h in ru d is ts and co lon ia l c o ra ls , in i ts upper p o rtio n . This

form ation is in turn o ve rla in by the interbedded sandstones and shales

of the Mojado Formation. The s im ila r it ie s between the upper beds of the

U-Bar Formation and the upper Mural Limestone in both lith o lo g y

( O rb ito lin a -bearing , ru d is t re e fs ) and age (E a rly A lb ian ) make th e ir

c o rre la tio n q u ite reasonable as suggested by Z e lle r (1965) and Hayes

(1970b). Hayes (1970b) co rre la ted the lower portion o f the U-Bar

Formation to the lower Mural Limestone and most o f the M orita Formation

(F igure 6) based m ainly on the above c o r re la la t io n . This c o rre la tio n

has importance as i t suggests th a t marine conditions p reva iled in south­

western New Mexico before they ex is ted in southeastern Arizona during

the E arly Cretaceous transgression in to the region (Hayes, 1970b).

Further evidence o f th is transgression can be found in lower

Cretaceous outcrops in the mountain ranges o f Trans-Pecos Texas. This

region marked the northeastern edge o f the Chihuahua Trough during E arly

Cretaceous tim e. In a study o f the geology o f the S ie rra Blanca area ,

A lb r itto n and Smith (1965) noted the probable c o rre la t io n o f the Late

Figure 6. C o rre la tio n chart - Cretaceous o f southeastern Arizona and southwestern New Mexico (a f te r Hayes, 1970b).

I I* PststoeieMountains

Muschwcs Mounts.ns and Canals

Hills

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(Ztiler. IM S . ZeMer and

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Cult coast r t f «en I

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MuralLimestone

Merita

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Figure 6

13

A p tian -E arly A lbian B lu ff Mesa Limestone to the Mural Limestone. The

B lu ff Mesa Limestone consists dominantly o f lim estone w ith lesser

amounts o f sandstone and shale. O rb ito lin a , ru d is ts , and coral

biostromes were observed in various limestone beds. A lb r itto n and Smith

also noted the occurrence o f ru d is t banks in the Campagrande Formation,

a local equ iva len t o f the B lu ff Mesa Limestone.

The B lu ff Formation, studied in the Eagle Mountains and v ic in i t y

(Underwood, 1975, 1980), is also o f Late Aptian - E a rly A lbian age. The

u n it consists o f interbedded sandstones and O rb ito lin a -bearing lim e­

stones. A f i f t y - f o o t r u d is t id - r e e f limestone bed was observed in the

form ation in the Lost V a lle y area.

The Quitman Formation, studied in the southern Quitman Mountains

(Jones and Reaser, 1970), is ye t another example o f interbedded e la s tic s

and carbonates deposited in the Chihuahua Trough during the Late Aptian

to E arly A lb ian . This form ation is s tr ik in g in i ts s im ila r ity to the

Mural Limestone considering the distance between the two. Of special

note are the numerous branching corals and coral heads found in the

lowermost member,and the commonly c l if f - fo rm in g uppermost member, r ic h

in O rb ito lin a and ru d is ts . The lower member o f the Mural contains a

zone o f massive lim estone beds w ith numerous branching and massive

corals in the Guadalupe Canyon area . In a d d it io n , the O rb ito lin a and

ru d is t-b e arin g upper Mural is a c l i f f - fo r m e r wherever exposed in

southeastern Arizona.

The equ iva len t Quitman, B lu f f , and B lu ff Mesa Limestone

formations o f Trans-Pecos Texas are a l l s im ila r in age (Late Aptian -

E arly A lb ian) and general lith o lo g y to the Mural Limestone and upper

portions o f the U-Bar Formation, and are most l ik e ly c o r re la t iv e .

One in te re s tin g observation is th a t w hile th is f i r s t major

transgression o f the sea in to the northern extremes o f the Chihuahua

Trough is recorded in these Late Aptian - E arly A lbian carbonates from

southeastern Arizona to Trans-Pecos Texas, la te r transgressive pulses

are recorded only in the outcrops o f Trans-Pecos Texas. While

successive episodes o f carbonate dep os ition , each spreading fu r th e r

northward and eastward (A lb r it to n and Smith, 1965), occurred in Trans-

Pecos Texas in to the Cenomanian, southeastern Arizona experienced

f lu v ia l - d e l t ia c sedim entation (C in tu ra Formation) and then non­

d ep o s itio n /ero s io n . Hayes (1970b) believed th is regression o f the sea

from southeastern Arizona a f te r E a rly Albian time and the successive,

northeastward spreading transgressions o f the sea in to Trans-Pecos

Texas in to the Cenomanian, were evidence o f s l ig h t reg ional northeastward

t i l t i n g . This regional t i l t i n g may have represented the e a r l ie s t

oncomings o f the i n i t i a l pulses o f the Laramide Orogeny in to south­

eastern Arizona when orogenic deformation and arc magmatism migrated

eastward in response to the decreased descent dip o f the subducted

Fara llon lith o s p h e ric p la te beneath the southern C o d ille ra .

In northeastern Sonora, Mexico, Warzeski (1979) traced the upper

Mural Limestone south over seventy k ilom eters from the ty p e -lo c a tio n near

14

15

Bisbee before the section is covered by Quaternary vo lcan ics . He

observed a progressive southward deepening o f w ater recorded in the

upper Mural as i t increased from f i f t y to close to three hundred meters

in thickness along th is tra v e rs e . He also described a general increase

in the s ize o f c o ra l-a lg a l- r u d is t patch reefs along th is traverse u n t i l

re e f banks up to e ig h t kilom eters across were encountered near an

ir re g u la r s h e lf margin in northeastern Sonora.

LOWER CRETACEOUS OF THE GUADALUPE CANYON AREA

Geologic S e ttin g

Lower Cretaceous rocks crop out along the southwestern f la n k o f

the P e lo n c illo Mountains in extreme southeastern Arizona. Cooper (1959)

designated the Lower Cretaceous in the area as Bisbee Group u n d iffe re n ­

t ia te d during reconnaissance mapping (1 :125 ,000 ) o f southeastern Cochise

County, Arizona (F igure 7 ) . Hayes' (1982) reconnaissance geologic map

of the area (1 :6 2 ,5 0 0 ) depicts the Lower Cretaceous a t the form ation

le v e l , but several o f his s tra tig ra p h ic and s tru c tu ra l in te rp re ta tio n s

are questionable. Two normal fa u lts located w ith in the Lower Cretaceous

on his map in c o rre c t ly denote which f a u l t block has been downdropped

r e la t iv e to the o ther. He mapped a la rge Lower Cretaceous outcrop in

sec. 17, T. 24 S . , R. 32 E . , as M orita Formation, but the section is

dominated by lower and upper Mural Limestone. He also in c o rre c t ly

mapped two small outcrops in sec. 3 , T . 24 S . , R. 32 E . , as C intura

Formation, which are upper Mural Limestone.

The major Bisbee Group outcrop in the area trends north -

northwest and is bounded on the northeast by the Baker Canyon F au lt

(F igure 7 ) . This fa u l t is p a rt o f a rin g fra c tu re system on the

southwest margin o f the Geronimo T r a i l cauldron (E rb , 1979). The

cauldron is f i l l e d w ith the t u f f o f Guadalupe Canyon which has been

dated (K-Ar) a t 24.2 ± 0 .5 m ill io n years (Deal e t a l . , 1978).

16

Figure 7. Reconnaissance geologic map (1 :1 2 5 ,0 0 0 ) o f study area ( a f t e r Cooper, 1959). The Lower Cretaceous is mapped as Bisbee Group u n d if fe re n tia te d .

________________________ Key________________________________

Qal - Quaternary a lluviumQb - Quaternary b as a ltT r - T e r t ia ry rh y o lite sKb - Bisbee Group, CretaceousPe - Epitaph Dolom ite, PermianPc - Colina Limestone, PermianPTPe - Earp Form ation, Penn.-PermianTPh - H o rq u illa Limestone, Pennsylvanian

V - - :

Figure 7

NE

W

ME

XI

18

S tra tig rap h y

Bisbee Group outcrops in the study area consis t o f the M orita

Formation and the Mural Limestone. The Glance Conglomerate and C intura

Formation are not exposed. Although the general l i th o lo g ic nature o f

the M orita Formation and the Mural Limestone in the area is s im ila r to

the type section near Bisbee, one im portant d iffe re n c e occurs. A group

of massive, cl i f f - fo rm in g , carbonate beds, averaging 9 to 12 m eters,

and lo c a l ly exceeding 24 meters in th ickness, is present w ith in the

lower member o f the Mural Limestone. These carbonate beds are very

s im ila r in th e ir massive, cl if f - fo rm in g nature to the upper member o f

the Mural Limestone, and in fa c t , were m istakenly c a lle d upper Mural

Limestone in two s tra tig ra p h ic sections measured by G. Grocock (1975) in

his study o f the u n it in southeastern Arizona. A composite section o f

the Mural Limestone in the study a rea , based on measured sections of the

author (Appendix A) and a re in te rp re ta tio n o f G. Grocock's sections

(1975 ), shows a to ta l thickness o f approxim ately 290 to 305 meters

(F igure 8 ) . The upper member o f the Mural is comparable in thickness to

the type sec tio n , but the lower member is approxim ately tw ice as th ic k .

The massive, c l i f f - fo r m in g , carbonate beds w ith in the lower

Mural also crop out 32 k ilom eters west o f the study area in the southern

Peri11a Mountains, where they have thinned to 6 meters in th ickness.

The carbonate beds have not been described a t the type s ec tio n , and are

in fe rre d to p inch-out between the southern Peri11a Mountains and the

type lo c a le , 43 k ilom eters to the west in the southern Mule Mountains.

19

100 m.

50 m.

25 m.

Figure 8 . Composite section o f the Mural Limestone in the Guadalupe Canyon area . W hile the upper Mural is com­parab le in thickness to the type s e c tio n , the lower Mural is approxim ately tw ice as th ic k . W ith in the lower Mural there is a group o f massive, c l i f f - fo r m in g carbonate beds l i t h o lo g ic a l ly s im ila r to the upper M ural.

20

The areal ex ten t o f these carbonate beds is not known, but they would be

expected to th icken southeastward in to the Chihuahua Trough. In fa c t ,

i t is possible th a t the carbonate beds and upper member o f the Mural

are two tongues th a t th icken and merge in to one u n it to the southeast

(F igure 9 ) . Although the two are p e tro g ra p h ic a lly d is tin g u is h a b le , they

are very s im ila r l i th o lo g ic a l ly in the study area . In a d d itio n ,

Warzeski (1979) states th a t the upper member o f the Mural Limestone

thickens from f i f t y meters a t the type section to n early 300 m eters,

south in Sonora, Mexico. The s tra tig ra p h ic separation between the two

in the study area is on ly 92 to 107 m eters, and thus, th e ir coalescing

in to a s ing le u n it to the south is not u n lik e ly .

S tructure

While very good exposures o f the upper Mural and the massive,

cl if f - fo rm in g , carbonate beds w ith in the lower Mural are present in

the study a rea , much o f the lower Mural is poorly exposed or covered.

The M orita Formation is also poorly exposed, except fo r an in te rv a l o f

interbedded carbonates and e la s t ic s near the top o f the u n it . This

in te rv a l is separated from the lower Mural Limestone by a c la s t ic

section approxim ately 30 to 61 meters th ic k . The exact thickness o f

th is c la s t ic section is d i f f i c u l t to ascerta in due to i ts poor exposure

and abundant fa u lt in g in the area . A map o f prominent Lower Cretaceous

carbonate outcrops (F igure 10) constructed as an o verlay on a 1:65,000

scale a e r ia l photograph shows major s tructu res in the Lower Cretaceous

in the study area . The map is based on prominent outcrops o f the three

Figure 9. Fence diagram - upper Mural Limestone:

1) The datum fo r the diagram is the top o f the upper Mural Limestone;i t is assumed to be a p lanar surface.

2) The a v a ila b le data fo r Sonora, Mexico show the upper Mural Limestoneto be approxim ately 300 meters th ic k , and thus, 300 meters is thea r b itr a r y c u t -o ff thickness fo r the diagram.

3) Data:

A) Sonora, Mexico, 70 km S-SE o f type section (B isbee, A Z ); upper Mural Limestone approxim ately 300 meters th ic k (W arzeski, 1 9 7 9 ).

B) Bisbee, A Z .; upper Mural Limestone approxim ately 50 meters th ic k (S c o tt, 1979); lower Mural Limestone averages 122 meters in thickness (Grocock, 1975).

C) Guadalupe Canyon area , A Z .; upper Mural Limestone approxim ately 50 meters th ic k (a u th o r); lower Mural Limestone approxim ately 245-260 meters th ic k (a u th o r); lower Mural includes a s tr in g e r of upper Mural (12 meters in average th ickness) approxim ately 92-107 meters below the main body o f the upper Mural (a u th o r ) .

D) Southern P e ri11 a Mountains, A Z .; upper Mural s tr in g e r in lower Mural averages 6 meters in thickness (a u th o r); o ther param eters assumed comparable to the Guadalupe Canyon area.

4) In te rp re ta tio n s :

A) The upper and lower Mural in te r f in g e r over la rg e d istances.

B) The dominant terrigenous in flu x in to the region is from the northwest.

C) A rapid thickening o f carbonates to the south-southeast occurs in the region.

z \

Figure 10. Map o f prominent Bisbee Group carbonate outcrops in the Guadalupe Canyon area . Map is con­structed as an overlay on a 1 :65,000 scale a e r ia l photograph.

Key

upper Mural Limestonemassive carbonates, lower Mural Limestonemixed c la s t ic and carbonate s e c tio n , M orita Formationnormal f a u l t , b a ll and bar on downthrown blockth ru s t f a u l t , sawteeth on o ve rrid in g blockplunging synclineplunging a n t ic lin es tr ik e and dip o f beddings tr ik e and dip o f overturned beddings tr ik e and dip o f v e r t ic a l bedding

22

Figure 10.

23

well-exposed s tra tig ra p h ic in te rv a ls discussed above, and thus, they are

h ig h lig h ted . I t should be noted th a t th is map shows only major trends,

and does not d e ta il man o f the s tru c tu ra l com plexities present in the

area.

The major north-northw est trending Bisbee Group outcrop contains

evidence fo r compressional and la te r tensional forces acting on the

area. The evidence fo r compressional forces ex is ts as th ru s t fa u lt in g

in the upper member o f the Mural Limestone, where section is repeated,

and in small and la rge scale fo ld in g (F igure 1 1 ). The compressional

forces acted in a d ire c tio n approxim ately normal to the trend o f the

outcrop (E-NE to W-SW) and may be re la te d to Laramide d e fo rm atio n /tec ­

tonics th a t a ffe c te d the region during Late C retaceous-Early T e r t ia ry

tim es. Many o f the compressional features are truncated by normal

fa u lts . Tensional forces in the area were a c tiv e in a d ire c tio n

approxim ately p a ra lle l to the trend o f the outcrop (N-NW to S-SE). The

tensional forces were probably re la te d to co llapse associated w ith the

Geronimo T ra il cauldron o f m id -T e rtia ry age.

24

Figure 11. Large, plunging sync l ine in the Lower Cretaceous, Guadalupe Canyon area . The c l i f f former a t the top o f the r idge is upper Mural Limestone.

PETROGRAPHY, DEPOSITIONAL ENVIRONMENTS AND DIAGENESIS

In trod uction

The three s tra tig ra p h ic in te rv a ls h ig h lig h ted on the o verlay

have been studied in d e ta il to gain in s ig h t in to th e ir respective depo-

s itio n a l and d iagenetic environments. T h e ir se lec tio n is based upon

good exposure, and dominance o f carbonate l ith o lo g ie s , which are much

more useful in d ica to rs o f depositional environment than terrigenous

s i l ic ic la s t ic s . The three in te rv a ls are: (1 ) mixed e la s t ic s and

carbonates w ith in the upper p ortio n of the M orita Formation, (2 ) a

group o f massive, cl i f f - fo r m in g , carbonate beds w ith in the lower Mural

Limestone, and (3 ) non -reefa l upper Mural Limestone.

The mixed e la s tic s and carbonates w ith in the M orita were

deposited in very shallow subtidal to sup ra tida l environments. The

in te rv a l contains the f i r s t s ig n if ic a n t amount o f carbonates deposited

by the E arly Cretaceous sea in the study area .

The massive carbonates w ith in the lower Mural Limestone in d ic a te

u su a lly low energy conditions in a shallow s h e lf environment ad jacent to

a high energy zone. They are im portant in th a t they represent the

f i r s t period o f s tab le carbonate deposition w ith no or l i t t l e t e r r i ­

genous in flu x in to the study area during the E arly Cretaceous tra n s ­

gression. As prev ious ly mentioned, the massive carbonates may in fa c t

be a s tr in g e r o f upper Mural w ith in the lower member. These f i r s t two

s tra tig ra p h ic in te rv a ls o f in te re s t are separated by a pulse o f c la s t ic

25

26

deposition (uppermost portion o f M orita Formation) and interbedded

e la s tic s and a rg illaceo u s carbonates o f the lower M ural.

Non-reefal upper Mural Limestone in the area re f le c ts low

energy conditions in a protected shallow s h e lf environment. The u n it

represents the major period o f s tab le carbonate deposition w ith no or

l i t t l e terrigenous in f lu x in to the study area - the height o f the E arly

Cretaceous transgression . Patch reefs occur in the study area , and have

been prev ious ly studied (S c o tt, 1979; Grocock, 1975). The massive

carbonate beds w ith in the lower member and the upper Mural Limestone are

also separated by interbedded e la s tic s and a rg illaceo u s carbonates

typ ica l o f the lower Mural Limestone.

Mixed C la s tic and Carbonate Section , M orita Formation

A great v a r ie ty of carbonate lith o lo g ie s are present w ith in the

upper portion o f the M orita Formation. O o sp arite s -o o m ic irites ,

p e ls p a rite s -p e lm ic r ite s , b io m ic rite s -b io m ic ru d ite s , and do lom icrites

dominate. Many o f the limestone beds have been s l ig h t ly to almost com­

p le te ly do lom itized . Fossil m ateria l in the beds consists dom inantly

of pelecypod, gastropod, and echinoderm fragments, as w ell as fo ra m in i-

fe ra , including m il io l id s . Rare bryozoan fragments and Codiacean algae

( Halimeda or Halimeda precursor (F igure 12 )) are also present. Very

f in e to f in e sand-sized terrigenous grains are lo c a l ly abundant. Quartz

is the dominant m in era l, w ith rare c h e rt, m ic ro c lin e , muscovite,

and c h lo r ite occurring . In several carbonate beds contain ing t e r r i ­

genous q u a rtz , the grains have developed la rg e , euhedral overgrowths

(F igure 13 ).

Figure 12. Codiacean a lg a l fragment (Halimeda or Halimeda precursor) (p la in l ig h t , 35 X ).

Figure 13. Terrigenous quartz gra in w ith complex h is to ry . Grain has undergone a t le a s t two and probably three cycles o f tran sp o rta tio n - deposition - cem entation, as evidenced by the abraded overgrowths (crossed n ic o ls , 500 X ) .

27

Figure 13.

28

Dolomite and do lom itic limestones are common throughout the

section . Many o f the do lom itic limestones are m ottled in appearance due

to the p a r t ia l replacement o f the limestone by dolom ite (F igure 1 4 ).

The d o lo m itiza tio n appears to have only a ffe c te d the reworked sediment

( i . e . , burrow f i l l s ) o f these prev ious ly b io turbated zones. This occur­

rence has been noted elsewhere in the s tra tig ra p h ic record , such as the

Lower Ordovician Tribes H i l l Formation o f e a s t-c e n tra l New York (Braun

and Friedman, 1969), but the reason fo r the s e le c tiv e d o lo m itiza tio n is

not d e f in i t iv e ly known. Probably, increased p e rm e a b ility due to the

reworking o f the sediment was high enough to a llow the la te r passage o f

do lom itiz ing f lu id s . Dolomite in the section can be subdivided in to two

morphological types. The f i r s t , and by fa r most abundant type , consists

of f in e ly to very f in e ly c ry s ta l l in e dolom ite w ith a xenotopic to

hyp id io top ic fa b r ic (F igure 1 5 ). The dolom ite c ry s ta ls are commonly

h em atite -s ta in ed . The second type is much less common and g e n e ra lly

associated w ith oosparites . I t consists o f is o la te d , euhedral rhombo-

hedrons ranging in s ize from 50 to 250 ym (F igure 1 6 ). These dolom ite

c ry s ta ls are also commonly h em atite -s ta in ed , and some show d is c re te

iro n -r ic h zonations (F igure 17 ).

Very l i t t l e p o ro s ity ex is ts w ith in the carbonate u n its o f the

M orita Formation in the study area . O r ig in a lly , many beds had e x c e lle n t

prim ary p o ro s ity , but a l l have been t ig h t ly cemented. Minor amounts o f

in t r a p a r t ic le p o ro s ity ex is ted w ith in the in te rn a l chambers o f fo ra m in i-

fe ra and w ith in c a lc if ie d worm tubes in several beds. Large amounts o f

Figure 14. M ottled dolom itic lim estone. The d o lo m itiza tio n has a ffe c te d only the reworked sediment ( i . e . , burrow f i l l s ) o f th is pre­v ious ly b ioturbated zone.

Figure 15. P a r t ia l ly dolom itized p e lm ic rite p e ls p a rite . Note the f in e c rys ta l s ize and xenotopic fa b r ic o f the dolom ite (crossed n ic o ls , 35X).

29

Figure 15.

Figure 16. Large, is o la te d , hem atite - sta ined , euhedral dolomite rhombohedron in a d o lo m itic , fo s s ilife ro u s oosparite (crossed n ic o ls , 100X).

Figure 17. Dolom itic o osparite . Note the concentration o f hem atite around the p e r i­meters o f the dolom ite rhombohedra. One rhombohedron shows a b e a u tifu l iron zonation w ith an iro n -r ic h center (crossed n ic o ls , 100X).

30

Figure 17

31

in te rp a r t ic le p o ro s ity ex isted w ith in h ig h ly p e lle te d muds ( in t e r -

p e l le ta l ) and w ith in o o l i t ic sand bodies ( in t e r o o l i t i c ) . The o o l i t ic

sand bodies developed ad d itio n a l secondary mol die p o ro s ity from the

so lu tion ing o f associated a ra g o n itic she ll m a te r ia l, as evidenced by

r e l ic t m ic r ite rims (F igure 1 8 ). The prim ary in te r o o l i t ic p o ro s ity and

secondary mol die p o ro s ity were both la te r f i l l e d w ith a coarse, equant,

mosaic sparry c a lc ite cement.

M ilky quartz c ry s ta ls and coarse, b lack , c a lc ite c ry s ta ls

are present in several beds w ith in the sec tio n . These c ry s ta ls are

u su a lly associated w ith unlam inated, m o ttled , or i r r e g u la r ly , f in e ly -

laminated (c ry p ta lg a l laminae?) m ic r i t ic u n its (F igures 19, 2 0 ). Many

of the quartz c ry s ta ls d is p la y a pseudofibrous e x tin c tio n (F igure 21)

very s im ila r to th a t described by Rubin and Friedman (1977) in quartz

cry s ta ls f i l l i n g vugs formed by the replacement o f s u lfa te nodules in

the Cambro-Ordovician W hitehall Formation o f eastern New York. Chowns

and E lkins (1974) describe the presence o f a comparable flamboyant

spectra l e x tin c tio n in quartz c ry ta ls in quartz geodes a f te r anhydrite

nodules in two m id-M ississippian formations in Tennessee. The quartz

c ry s ta ls are in c lu s io n -r ic h and x -ra y d if f r a c t io n ana lys is o f a powdered

sample o f the c ry s ta ls ind ica tes th a t the inclusions are an h yd rite .

These p roperties provide convincing evidence th a t the quartz c ry s ta ls

and associated c a lc ite c ry s ta ls have in fa c t replaced former anhydrite

c ry s ta ls in the sec tio n . Adjacent to a m ic r ite bed contain ing many o f

the c a lc ite c ry s ta ls is a l i th o lo g ic a l ly s im ila r u n it w ith very w ell

Figure 18. F o s s ilife ro u s oosparite . Note the r e l ic t m ic rite rims o u tlin in g many o r ig in a l shell fragm ents. The shell molds, l ik e the o rig in a l abundant in te rp a r t ic le p o ro s ity , have been f i l l e d w ith a coarse, equant, mosaic, sparry cal c ite cement (crossed n ic o ls , 35X).

Figure 19. Authigenic quartz a f te r anh ydrite . Note the ir re g u la r lam inations in the sample - c ryp ta lg a l laminae?

32

Figure 18.

Figure 19.

Figure 20. Authigenic quartz a f te r anh ydrite . Abundant anhydrite inclusions give the quartz i ts m ilky co lo r.

Figure 21. Pseudofibrous e x tin c tio n in auth igenic quartz c rys ta l a f te r anhy­d r i te . Note the numerous inclusions in the c rys ta l and i ts ra d ia tin g hab it (crossed n ic o ls , 35X).

33

Figure 21.

developed vuggy p o ro s ity (F igure 2 2 ). The vuggy p o ro s ity most l ik e ly

represents the d isso lu tio n o f unreplaced anhydrite c ry s ta ls w ith in the

bed. This lo c a liz e d secondary p o ro s ity development was the only

observed p o ro s ity s t i l l e x is tin g w ith in the sec tion .

L o c a lly , carbonates w ith in the section show signs o f compaction

and pressure s o lu tio n . One o o l i t ic bed shows so lu tio n along gra in con­

tac ts (F igure 23) and s t y le l i t e s are common in several un its (F igure

2 4 ). Pressure s o lu tio n , l ik e cem entation, is a p o ro s ity d es tru c tive

process. I t not only decreases or e lim inates pore volumes a t the s ite s

of d is s o lu tio n , but the dissolved m ateria l may be re p re c ip ita te d as

cement lo c a lly or re g io n a lly , fu r th e r destroying p o ro s ity .

Sandstones in and adjacent to the section are c la s s if ie d as

quartzaren ites and g la u c o n itic q u a rtza re n ite s . An average o f three

point counts gives a q u a r tz - fe ld s p a r -1 ith ic fragment ra t io o f 9 7 -1 -2 .

Very minor amounts o f terrigenous p la g io c lase , m ic ro c lin e , c h e rt,

muscovite, and b io t i te ( lo c a l ly a lte r in g to c h lo r ite ) are present. At

the outcrop, the sandstones w ith abundant in t e r s t i t i a l g lauconite be­

come h e a v ily h em atite -s ta ined as ferrous iro n is freed from the g lau­

con ite m ineral la t t ic e and ox id ized to form hem atite in the surface

weathering environment. The sandstones average f in e to very f in e sand­

s ized . The q u artzaren ites are w e ll-s o rte d and g ra in roundness ranges

from sub-rounded to w ell-rounded. T e x tu ra lly these rocks are c la s s if ie d

as supermature. The g la u c o n itic q u a rtza ren ites are only m oderately-

sorted and gra in roundness ranges from subangular to rounded.

34

Figure 22. Vuggy p o ro s ity in a m ic r i t ic lim e­stone. The vugs most l ik e ly developed due to the p re fe re n tia l leaching o f anhydrite c ry s ta ls in the carbonate sediment by freshw ater.

Figure 23. Pressure so lu tion ing in a do lom itic o osparite . Note the crenulated contacts between many o f the ooids. Although not as g re a tly a ffe c te d , some o f the dolomite rhombohedra have crenulated borders, suggesting th a t do lo- m itiz a tio n occurred before the pressure so lu tio n in g . Several o f the dolom ite rhombo­hedra are iro n zoned (crossed n ic o ls , 100X).

35

Figure 23

36

Figure 24. S t y l o ! i t e s in a f o s s i l i f e r o u s o o s p a r i te -o o m ic r i te . Note t ru n ca t ion o f ooids and she l l fragments at s t y l o ! i t e contacts (crossed n ic o l s , 35X).

37

T e x tu ra lly these rocks are c la s s if ie d as submature. The sandstones

studied contained no observable p o ro s ity . Pressure so lu tio n and

cementation have y ie lded t ig h t rocks. Quartz overgrowths are the major

form o f cem entation, although patches of carbonate cement are also

present. Grain contacts are m ostly s tra ig h t , but concavo-convex and

rare sutured contacts are also found. The la te r two types in d ic a te

pressure so lu tio n .

The l i th o lo g ie s , sedimentary s tru c tu re s , and d iagen etic features

present in the section c le a r ly in d ic a te very shallow sub tida l to supra-

t id a l deposition in a f a i r l y a r id environment. The depositional

environment was very s im ila r to th a t seen a t present day Abu Dhabi on

the Persian G u lf or in Permian s tra ta on the Northwestern S helf o f the

Delaware Basin o f West Texas. A hypothetical carbonate-evaporite

sequence derived by James (1979) (F igure 25) provides a close working

model fo r the mixed carbonate and c la s t ic section in the M orita

Formation. The three major v a ria tio n s from the model are: (1 ) the

presence o f s i l ic ic la s t ic s in the sec tio n , (2 ) the lack o f abundant

evap o rites , and (3 ) evidence o f e a r ly , m eteoric w ater d iagenesis,

in d ic a tin g th a t hypersaline conditions were not always present in the

environment.

B io m icrites -b io m icru d ites and p e lm ic r ite s -p e ls p a r ite s are common

in the section and represent low-energy, sub tida l con d itio ns . The

common occurrence o f m ottled tex tu res and the p e l le t - r ic h zones in d ic a te

th a t burrowers were a c tiv e in the sediment column.

38

CARBONATE- EVAPORITE SEQUENCE

CROSS-SEDOED SMtDSTOm

S1LTT OOLOWTE N 0 0 U .4 * "V ANHYDRITE

CARBONATE- EVAPORITE SEQUENCE

LEACHED BY FRESH WATER

<vENTEROUTHIC

NODULES OR C R Y S TA LS ^ANHYDRITE

SKELETAL LIME SAND

FOSSILIFEROUS BURROWED AND BlOTwRRATED LIME

MUDSTONE

-ARCILLACEOUS L .mESTONE —

.SK E LE TA LIN TR A C LA S T UM £ SAND OR CONGLOMERATE -

COLLAPSED BRECCIA MARINE LIMESTONE a n o / o r SANDSTONE

. c o l l a p s e d ANMYORITIC DOLOMITE - LEACHEDa n o m y o r ite n o d u le s

Figure 25. Id e a lize d carbonate-evaporite sequence (shallow subtidal to sup ra tida l conditions) ( a f te r James, 1979).

39

Numerous one to four fo o t th ic k , trough c r o s s -s t r a t i f ie d , f o s s i l -

iferous oosparite beds occur and represent o o l i t ic sand bodies th a t were

s h ifte d across shallow , s u b tid a l, m ic r i t ic s h e lf sediments by t id a l

curren ts . A v a r ie ty o f f ie ld re la tio n s and sedimentary structures

support th is in te r t id a l environment in te rp re ta t io n : (1 ) the c lean ,

c ro s s -s tra t if ie d carbonate sands in d ic a te a winnowing o f mud and rework­

ing of sand-sized m ateria l by curren t a c t iv i t y ; (2 ) the upper portions

of a p e lm ic rite bed show signs o f scour and reworking, and the basal

portion o f the overly ing o o l i t ic bed contains in tra c la s ts derived from

the p e lm ic rite u n it , in d ic a tin g th a t p e rio d ic progradation o f o o l i t ic

sand bodies across s u b tid a l, m ic r i t ic , s h e lf sediments occurred; (3 ) an

o o l i t ic bed is overlayen by s tro m a to lite s , suggesting th a t p erio d ic

shallowing o f w ater and s ta b il iz a t io n of p rev ious ly m obile , carbonate,

t id a l sand bodies by blue-green algae occurred; and (4 ) the upper portion

of an o o l i t ic bed has been ripped-up and the derived in tra c la s ts in ­

corporated in the overly ing u n it (F igure 2 6 ) , in d ic a tin g th a t some car­

bonate, t id a l sand bodies underwent a t le a s t p a r t ia l 1i th i f ic a t io n w ith

la te r erosion by tid a l/s to rm ? cu rren ts . Some of the oosparites lack

c ro s s -s tra t if ic a t io n and have a m ottled te x tu re , suggesting th a t the

carbonate, t id a l sand bodies, l ik e the m ic r i t ic sub tida l sediments, were

burrowed. One bed in p a r t ic u la r shows d is c re te burrow f i l l s in which

the sediment o f the in f i l l in g s has been p r e fe r e n t ia l ly s i l i c i f i e d

(F igure 2 7 ).

Several th in (1 to 2 fe e t th ic k ) sandstone beds are found w ith in

the section . These q u artza ren ites are te x tu r a l ly supermature, and o ften

Figure 26. Scour surface , and in tra c la s ts derived from a d o lo m itic , fo s s ilife ro u s oosparite .

Figure 27. P a rt ia l s i l i c i f i c a t io n o f burrow- f i l l sediment. The ooids are replaced by m ic ro c ry s ta llin e quartz w hile the m atrix is g en era lly replaced by chalcedony (crossed n ic o ls , 35X).

40

Figure 26.

Figure 27.

41

contain burrows, small scale trough and p la n a r-ta b u la r cross­

s t r a t i f ic a t io n . These beds represent the terrigenous eq u iva len t o f the

o o l i t ic sand bodies. As the M orita carbonate beds represent the f i r s t

carbonate deposition in the study area during the E a rly Cretaceous, i t

is not su rp ris ing th a t terrigenous in flu x in to the environment s t i l l

p e r io d ic a lly occurred, as evidenced by the interbedded sandstone un its

in the section .

M ic r i t ic and s tro m a to lit ic un its (F ig ure 2 8 ) , some contain ing

quartz and/or c a lc ite c ry s ta ls a f te r an h yd rite , represent upper in te r ­

t id a l to sup ra tida l con d itio ns . The presence o f anhydrite c ry s ta ls

in the sediment and abundant a lg a l mats suggest th a t the c lim ate in the

area was g en era lly a r id . Z e l le r (1965) describes two th ic k gypsum beds

(6 and 15 meters) in the oyster lim estone member o f the c o rre la t iv e U-

Bar Formation in the Big Hatchet Mountains to the east o f the study

area , fu r th e r supporting th is in fe ren ce . The upper growth l im i t o f the

a lg a l mats is co n tro lle d by the frequency o f floo d ing o f sup ra tida l

f la t s , w h ile the lower l im i t is co n tro lle d by w ater s a l in i t y . U n like

i ts predators, such as the gastropod C erith ium , blue-green algae can

to le ra te hypersaline conditions (James, 1979). In a r id c lim a tic

s e ttin g s , the upper in te r t id a l zone is most conducive to blue-green

algae growth.

I t is not su rp ris ing th a t dolom ite and do lo m itic limestones are

present in the section considering the depositional environment and a r id

clim ate th a t ex is te d . The evidence fo r former anh ydrite c ry s ta ls has

42

Figure 28. S tro m a to l i te s .

43

been p rev ious ly discussed. With high evaporation ra tes in the environ­

ment, evaporites p re c ip ita te d from hypersaline sea-w aters, removing

calcium , and increasing the magnesium-calcium r a t io o f the seawater.

The magnesium-rich brines generated then reacted w ith calcium carbonate

sediments to form dolom ite. As mentioned e a r l ie r , the vast m a jo rity o f

dolom ite in the section is f in e ly to very f in e ly c r y s ta ll in e w ith a

xenotopic to hyp id io top ic fa b r ic , and th is c lo s e ly matches the morpho­

lo g ic a l dolom ite type th a t forms under hypersaline conditions (Fo lk and

Land, 1975). With high s a l in i t ie s , dolom ite c ry s ta ll iz e s very ra p id ly

and forms anhedral mosaics o f very f in e c ry s ta ls . Subtidal sed i­

ments appear to be the most conmonly d o lo m itized , favoring a seepage

re flu x io n model o f d o lo m itiz a tio n , but evaporative pum ping-cap illary

concentration could also have been a c tiv e processes.

Although the section very c lo s e ly matches the c la ss ic model of

an a r id c lim a tic carbonate-evaporite sequence ( i . e . , hypersaline

dolom ites, former an h yd rite , abundant s tro m a to lite s , abundant o o l i te s ) ,

several features in d ic a te th a t fresh water was occassionally present

w ith in the environment. As discussed e a r l ie r , the o r ig in a lly high

p o ro s ity o o l i t ic t id a l sand bodies have been cemented w ith a coarse,

equant, mosaic sparry cal c i te cement. Associated a ra g o n itic she ll

m ateria l has been leached as evidenced by r e l i c t m ic r ite rim s, and the

re s u ltin g molds also f i l l e d w ith an equant, mosaic sparry cal c i te

cement. The coarse, equant, mosaic fa b r ic o f the cement, as w ell as the

leaching o f a ra g o n itic she ll m ateria l is c h a ra c te r is t ic o f a freshw ater

44

phrea tic d iagenetic environment (Longman, 1980). In a d d itio n , the

second morphological dolom ite type described e a r l ie r , u s u a lly occurs

w ith oosparites . This dolom ite consists o f is o la te d , la rge (50-250 pm),

euhedral rhombohedrons, s im ila r to the morphology of dolom ite produced

through seaw ater-freshw ater mixing in sch izohaline environments (Folk

and Land, 1975). Rapid in f lu x o f freshw ater in to a seawater system

w il l g re a tly reduce s a l in i t y w hile m aintain ing a r e la t iv e ly constant

magnesium-calcium r a t io , a llow ing slow dolom ite c r y s ta l l iz a t io n in the

form o f la rg e , euhedral rhombohedrons. Freshwater most l ik e ly moved

in to the environment through a groundwater system, but p erio d ic d ire c t

r a in fa l l could have been another source. Possib ly the association o f

dolomite in d ic a tiv e of freshw ater-seaw ater mixing w ith the oosparites

can be re la te d to the o r ig in a l high p e rm ea b ility and p o ro s ity o f the

o o l i t ic sand bodies being conducive to groundwater passage. Although

the importance o f fresh groundwater as a d iagen etic agent in a sabkha-

type environment is s t i l l disputed (Friedman, 1980), petrographic

c r i t e r ia in the carbonates o f the M orita Formation suggest th a t i t was

present. Many o f the d iagenetic features present in the M orita

carbonates in the study area have been described in Late P alezo ic rocks

of Bear Is la n d , Svalbard by Folk and Siedlecka (1 9 7 4 ), and are

considered d iagnostic o f a sch izohaline environment.

Evidence o f s i l ic a diagenesis is abundant in the carbonates

w ith in the sec tion . S i l ic i f ic a t io n of fo s s il m a te r ia l, such as worm

tubes (F igure 2 9 ), by chalcedonic and m ic ro c ry s ta llin e quartz is common.

Some she ll m ateria l has been replaced by bundles o f euhedral megaquartz

c ry s ta ls , although the reason fo r th is in te re s tin g form o f replacement

is unknown. In several fo s s ilife ro u s oosparite beds w ith in the sec tio n ,

the o o lite s have been p r e fe r e n t ia l ly s i l i c i f i e d over the in te rp a r t ic le

sparry c a lc ite cement. The replacement product is m ic ro c ry s ta llin e

quartz and in most instances the o r ig in a l tan g en tia l fa b r ic o f the

o o lite s has been preserved. SEM photos of an etched, slabbed surface

show the replacement fa b r ic q u ite w ell (F igure 3 0 ).

A poorly exposed c la s t ic sec tio n , approxim ately 30 to 61 meters

th ic k , is s itu a ted between the mixed c la s t ic and carbonate section in

the M orita Formation and the e la s t ic s and a rg illaceo u s carbonates o f the

lower member o f the Mural Limestone. Although not studied in d e t a i l , the

section appears to be dominated by the q u artza ren ites and g la u c o n itic

quartzaren ites described e a r l ie r . P e tr if ie d wood is commonly associated

w ith the g la u c o n itic q u a rtza re n ite s . Sedimentary s tructures in d ic a tiv e

of a beach shoreface environment, such as low angle c r o s s -s tr a t if ic a ­

t io n , heavy m ineral lam inae, and bedding plane p a rtin g lin e a tio n s , are

lo c a lly present in the q u a rtza re n ite s . T e x tu ra lly , the q u artzaren ites

are supermature, and th is is consis ten t w ith th e ir probable beach

shoreface environment o f deposition . The g la u c o n itic q u artza ren ties

are te x tu r a lly submature. The ir lack o f good s o rtin g , as w ell as

th e ir abundant in t e r s t i t i a l g lauconite in d ic a te deposition in a lower

energy and more reducing environment than the beach shoreface

45

Figure 29. C a lc if ie d worm tubes in m ien"tic sediment. Note s i l ic i f ic a t io n o f m ic r ite w ith in and adjacent to the tubes. Chalcedony is the dominant replacement product (crossed n ic o ls , 35X).

Figure 30. Slabbed and etched surface o f a fo s s ilife ro u s oosparite . The o o lite s have been p re fe re n t ia lly s i l i c i f i e d and th e ir o rig in a l tangen tia l fa b r ic preserved. Note the coarse sparry cal c ite rep lacing the she ll fragment (SEM, 25X).

46

Figure 29.

Figure 30.

q u a rtza ren ites . Possib ly they represent deposition in deeper waters

fa r th e r offshore from a b a r r ie r beach complex or tru e mainland beach.

Massive Carbonates, Lower Mural Limestone

The massive carbonate beds o f the lower Mural Limestone in the

study area are composed dom inantly o f fo s s ilife ro u s in tra m ic r ite s ;

o n c o lit ic , in t r a c la s t ic , oyster b iom icru d ites; and fo s s ili fe ro u s ,

o n c o lit ic in tra m ic r ite s to fo s s ili fe ro u s , in t r a c la s t ic , o n c o lit ic

m ic rite s . Rare ooids (less than 10 percent) occur in several beds.

Very f in e sand-sized terrigenous quartz is common in most o f the beds

and lo c a l ly is abundant. Some grains have euhedral overgrowths. In the

in te rv a ls where best developed, the onco lites average 3 .0 -4 .0 mm in

diameter and some reach 8 .0 -9 .0 mm (Figure 3 1 ). The onco lites u su a lly

consist o f a sh e ll fragment nucleus, w ith concentric coatings o f m ic r ite

bound by the filam entous algae G irv a n e lla . The algae have the form of

sim ple, non-branching tubu les , w ith no in te rn a l p a r t it io n s . The tubules

average 5 .0 -7 .5 urn in diameter (F igure 3 2 ). In tra c la s ts w ith in the beds

have a biomodal s ize d is tr ib u t io n w ith a prominent mode a t 1 .0 -2 .0 mm

and a lesser one a t approxim ately 0.25 mm. A v a r ie ty o f in tra c la s t

types are present (based on the nomenclature o f W ilson, (1 9 6 7 )) ,

inc lud ing: small in tra c la s ts (g e n e ra lly less than 0 .25 mm lo n g ); coated

s h e lls ; and b o try o id a l, eroded, and amorphous lumps ( a l l g e n e ra lly

g rea te r than 0 .40 mm across) (F igure 3 3 ). The few ooids observed have a

tan g en tia l fa b r ic and average 0 .50 mm in d iam eter.

47

Figure 31. Slabbed surface o f an o n c o lit ic lim estone. The onco lites u su a lly consist o f a she ll fragment nucleus w ith concentric coatings o f m ic r ite , bound by the filam entous algae G irv a n e lla .

Figure 32. G irvan e lla tubules w ith in an o n c o lite . The tubules are c y l in d r ic a l , lacking in te rn a l p a rt it io n s (p la in l ig h t , 250X).

48

Figure 32.

49

Abraded mol Tuscan she ll d eb ris , including h igh -sp ired gastro ­

pods, and echinoderm fragments are common throughout the sec tion .

Sm aller amounts o f m il io l id s , b is e r ia l fo ra m in ife ra , and bryozoan fra g ­

ments also occur. Several beds contain abundant, la rg e oyster she ll

fragments. The lower h a lf o f the section contains abundant corals and

includes three dominant types: (1 ) a hem ispheroidal, cero id coral

(A c tin as trea? ) ; (2 ) a la rg e , branching, ceroid c o ra l; and, (3 ) a sm all,

branching, phaceloid coral ( Calam ophyllia?) (F igures 34, 3 5 ). A ll

three are u su a lly ex ten s ive ly bored and not in l i f e p o s itio n .

Calam ophyllia? commonly has growth lin e s (F igure 3 6 ). These are a

product o f v a r ia b i l i t y in growth ra te , which is g re a t ly a ffe c te d by

water temperature (Weber e t a l . , 1975).

The rocks throughout the section are m ic r i te - r ic h , although re -

c ry s ta l l iz a t io n o f the m atrix and in tra c la s ts to microspar and

pseudospar is common. Several echinoderm fragments have syn tax ia l

growths developed around them due to replacement o f adjacent m ic r ite

m atrix . No prim ary or secondary p o ro s ity ex is ts w ith in the sec tion . A

very small amount o f prim ary in te rp a r t ic le p o ro s ity ex is ted lo c a lly

w ith in some o f the gra in -supported , o n c o lit ic m ic r ite s , but has been

f i l l e d w ith an equant, mosaic, sparry c a lc ite cement. The most note­

worthy te x tu ra l aspect o f the rocks is the presence o f abundant

allochems in d ic a tiv e o f high energy conditions or events (oo ids , c o ra ls ,

in tra c la s ts , large o n c o lite s ) w ith in a m ic r ite m a trix .

Figure 33. Botryoidal lumps in a fo s s ilife ro u s in tra m ic r ite . Note the ooids incorporated w ith in several o f the in tra c la s ts (crossed n ic o ls , 35X).

Figure 34. Abundant corals in m ic r ite -r ic h sediment. Zone w ith massive, ceroid corals (A ctinastrea?) is o verla in by zone w ith la rg e , branching, ceroid co ra ls .

50

Figure 34

Figure 35. Overturned, hem ispheroidal, ceroid coral (A c tin as tre a? ). Note numerous m ic r i t e - f i l le d borings.

Figure 36. Growth lin e s and m ic r i t e - f i l le d borings in sm all, branching, phaceloid coral (C alam ophyllia?).

51

Figure 36

52

The m ic r ite -r ic h nature o f the rocks w ith in the section denotes

deposition in a g e n e ra lly low energy environment. The numerous m a trix -

supported fa b ric s and s c a rc ity o f o r ig in a l prim ary in te r p a r t ic le

poros ity w ith in grain-supported fa b r ic s , in d ic a te th a t the te x tu ra l in ­

version observed in the rocks is most l ik e ly due to the p erio d ic tra n s ­

p o rta tio n o f the high energy allochems in to and w ith in the u s u a lly low

energy environment ra th e r than la te r m ic r ite i n f i l l i n g o f a high energy

lag deposit. Many o f the rocks re lease a s lig h t hydrogen s u lf id e odor on

fresh breaks, in d ic a tin g th a t lo c a l ly the environment was depleted

enough in fre e oxygen to a llow s u lfa te reduction to occur.

The lower h a lf o f the section is dominated by fo s s ilife ro u s

in tra m ic r ite s containing abundant corals and rare ooids. The corals are

not in l i f e p os ition and thus appear to have undergone loca l tra n s p o rt.

N either the corals nor ooids are c h a ra c te r is t ic o f low energy, m ic r ite -

rich environments, and both were probably washed in to the area from an

adjacent high energy zone. The in tra c la s ts , although in d ic a tiv e o f a

high-energy event, were derived from a m ic r ite -r ic h environment.

Storm-generated currents may have ca rried the corals and ooids land­

ward from a high-energy zone, and may also have been responsible fo r the

in tra c la s t form ation by causing loca l scour and rip -u p o f m ic r i t ic

sediments in the low-energy environment. The c o ra l-r ic h beds are

separated by very th in m ottled zones which appear to be the product o f

b io tu rb a tio n . The m ottled beds are v a ria b le in th ickness, each w ith an

i r r e g u la r , lower-bounding surface o f reworking. Burrowing a c t iv i t y

apparently varied in in te n s ity w ith in the environment.

53

The upper h a lf o f the section is dominated by fo s s ili fe ro u s ,

o n c o lit ic in tra m ic r ite s to fo s s ili fe ro u s , in t r a c la s t ic , o n c o lit ic

m ic rite s . Very rare ooids and coral fragments occur. The o n c o lit ic

m ic rites are g en era lly gra in -supported , but lack any sedimentary s tru c ­

tures in d ic a tiv e o f frequent cu rren t reworking. Although perio d ic

currents are needed to a llow the concentric growth o f the o n c o lite s ,

these allochems are a product o f a u su a lly low energy, m ic r ite -r ic h en­

vironment. The blue-green algae G irvan e lla thought to be responsible

fo r the o n c o lite development, is g e n era lly found in very shallow water

environments, commonly in te r t id a l zones (Horowitz and P o tte r , 1971).

An upward traverse through the massive carbonate section docu­

ments a probable decrease in the water depth o f the environment. The

sudden appearance o f onco lites and s c a rc ity o f corals in the upper h a lf

of the section is most l ik e ly an in d ic a tio n o f an abrupt change in one

or more environmental conditions - possib ly a subtidal to in te r t id a l

depth t ra n s it io n . The decrease in water depth hindered the tran sp o rta ­

tio n o f corals from a nearby high-energy zone in to the environment,

and probable increased s a l in i t ie s allowed the blue-green algae

G irvan e lla to f lo u ris h as conditions became unfavorable fo r i t s

predators.

As p rev ious ly mentioned, the massive carbonate beds o f the lower

Mural th in from an average o f 12 meters in the study area to 6 meters in

the southern P e r i l la Mountains, 32 k ilom eters to the west. Compared to

the study a rea , the southern P e r i l la Mountain section has fewer and

less-developed o n c o lite s , more abundant oyster sh e ll fragm ents, and

54

extrem ely rare coral fragments. A lso, beds w ith in the section lo c a l ly

have a m ottled tex tu re due to the p a r t ia l d o lo m itiza tio n o f lim estone.

These trends, as w ell as the o ve ra ll th inn ing o f the s ec tio n , in d ic a te a

landward progression to the west through a shallow , low energy, subtidal

environment, and thus, the in fe rre d high energy zone is believed to be

located to the southeast o f the study area .

P a rt ia l s i l i c i f i c a t io n o f oyster sh e ll debris is common w ith in

the section . Chalcedony is the dominant replacement product. Many

terrigenous quartz grains have euhedral overgrowths. Some o f these

overgrowths show corrosion and replacement by carbonate cement,

in d ic a tin g flu c tu a tio n s in the pH o f the d iagen etic environment.

Non-Reefal Upper Mural Limestone

The non-reefa l upper Mural Limestone in the Guadalupe Canyon

area is dominated by fo s s ilife ro u s m ic r ite s , sparse b io m ic rite s -

b iom icru d ites , and packed b io m ic rite s -b io m ic ru d ite s . Commonly these

m ic r ite -r ic h rocks have been lo c a l ly re c ry s ta ll iz e d to m icrospar and

even pseudospar. Although fo s s il m ateria l comprises the vast m a jo r ity

of allochem s, p e lle ts are lo c a l ly abundant, and rare in tra c la s ts are

also present. Small amounts o f f in e sand to coarse s i l t - s iz e d

terrigenous quartz occur in many o f the beds.

O rb ito lin a , m il io l id s , b is e r ia l fo rm in ife ra , and very abrad­

ed, coarse s i l t to coarse sand-sized echinoderm fragments and molluscan

she ll debris are common throughout the s ec tio n , and compose the f o s s i l -

ife rou s m ic rite s and sparse to packed b io m ic rite s . Beds r ic h in

55

oysters , ribbed pelecypods, ru d is ts (caprin id s and m onopleurids), and

gastropods comprise the sparse to packed b iom icru d ites .

No or l i t t l e p o ro s ity e x is ts in the non-reefa l upper Mural

Limestone in the study area . O rig in a l prim ary p o ro s ity ex is ted most

commonly as in tra s k e le ta l p o ro s ity w ith in the tes ts o f O rb ito lin a ,

m il io l id s , and b is e r ia l fo ra m in ife ra , but the vast m a jo r ity has been

f i l l e d w ith an equant, mosaic, sparry c a lc ite cement (F igure 3 7 ). Rare

in te rp a r t ic le p o ro s ity ex is ted lo c a l ly in zones o f packed s k e le ta l

debris ( in te r s k e le ta l ) , and in zones o f abundant p e l le t accumulation

( in t e r p e l le t a l ) (F igure 3 8 ), but th is pore space also has been f i l l e d

w ith an equant, mosaic, sparry c a lc ite cement. In a few instances

syn tax ia l cement around echinoderm fragments was observed.

The non-reefa l upper Mural beds in the study area represent a

low energy environment as evidenced by th e ir abundance o f m ic r ite ,

general p o o r-s o rtin g , and lack o f s ig n if ic a n t prim ary p o ro s ity . Many o f

the rocks in the section are f a i r l y f e t id , re leas in g a hydrogen s u lf id e

odor from fresh breaks. Several beds contain small patches o f 1im onite

pseudomorph a f te r p y r ite cubes. The fe t id nature o f the rocks, as w ell

as the loca l presence o f o r ig in a l p y r ite suggest th a t the environment was

a t times d e f ic ie n t enough in fre e oxygen to a llow s u lfa te reduction to

occur. Oxygen le v e ls , however, were never unfavorable enough to prevent

a la rge b io ta from th r iv in g as evidenced by the many fo s s i l - r ic h beds,

lo c a l ly p e l le t - r ic h zones, and many m ottled (b io tu rb a ted ) beds. E v i­

dence fo r occasional weak currents in the environment includes crude

Figure 37. Sparry cal c i te f i l l i n g prim ary in tra s k e le ta l p o ro s ity in an O rb ito lin a te s t (crossed n ic o ls , 35X).

Figure 38. Sparry c a lc ite f i l l i n g prim ary in te rp e lle ta l p o ro s ity . Adjacent she ll has a prism atic m icrostructure (crossed n ic o ls , 35X).

56

Figure 37

Figure 38.

57

horizon ta l s t r a t i f ic a t io n of fo s s il m ateria l in several beds (although

th is e f fe c t may be in p a rt due to p o s t-b u ria l com paction), and th in

bedding breaks in several un its consisting o f c ru d e ly -so rted , cru d e ly -

laminated fo s s il deb ris . The debris may represent s l ig h t scour and

reworking o f surface sediment forming extrem ely th in "lag" deposits .

Two beds consist o f oyster packstone in which the oysters are o riented

p a ra lle l to bedding and u su a lly concave-up. This apparent crude

horizon ta l s t r a t i f ic a t io n is not a product o f p erio d ic weak cu rren t

a c t iv i t y in the environment, as the concave-up o r ie n ta tio n o f these

oysters is hydrodynam ically unstab le. The beds probably represent very

lo w -r e l ie f oyster biostromes w ith the oysters in l i f e p o s itio n . The

section shows a general trend from fo s s ilife ro u s m ic rite s and sparse

b io m ic rites grading up in to cru d e ly , h o r iz o n ta l ly -s t r a t i f ie d b io -

mi crud ites and m ottled b io m ic rites suggesting a sub tle change in water

depth - a s l ig h t shallowing o f water w ith accompanied higher fre e oxygen

le ve ls and occasional weak bottom cu rren ts .

This environmental in te rp re ta t io n agrees w ith Grocock's (1975)

in ter-b ioherm m ic r i t ic fa c ie s or S c o tt's (1979) m o llu s k -m ilio lid -

o r b ito lin id fa c ie s . The low energy fac ies in the Guadalupe Canyon area

would be expected w ith the upper Mural s h e lf edge (and associated large

c o r a l-a lg a l- r u d is t banks) located many kilom eters to the south in

Sonora, Mexico (W arzeski, 1979). The s h e lf margin banks and wide s h e lf

would e f fe c t iv e ly in h ib it wave energy from reaching the study area .

Shoaling around patch reefs on the g e n e ra lly low energy s h e lf could

58

produce local high energy fac ies ( i . e . , p e lo id -o o id fa c ie s o f Scott

(1 9 7 9 )) .

One of the most common d iagenetic features observed in the upper

Mural section is the p a r t ia l s i l i c i f i c a t io n o f fo s s il m a te r ia l.

Chalcedony is the usual replacement product, but o ccas iona lly euhedral

megaquartz c ry s ta ls are seen. In some instances fo s s il m ateria l is

s i l i c i f i e d w ith the associated l iv in g c a v it ie s f i l l e d w ith coarse,

equant, sparry c a lc i te . Several horizons w ith in the section contain

i r r e g u la r ly shaped chert nodules which appear to be rep lac ing burrows

(F igure 3 9 ). They are composed dom inantly o f m ic ro c ry s ta llin e quartz

with small patches o f chalcedony. Around the cen ter o f each nodule

there is a t ra n s it io n zone in which there has been on ly p a r t ia l

replacement o f carbonate. The p re fe re n tia l s i l i c i f i c a t io n o f the

burrows may be re la te d to the presence o f associated organic m atter.

O xidation of the organic m atter could produce a loca l drop in pH,

favoring carbonate d isso lu tio n and s i l ic a p re c ip ita t io n , or the organic

m atter may ac t as a tem plate fo r s i l ic a nu c lea tio n . Late sparry

c a lc i t e - f i l l e d fra c tu re s are found throughout much o f the section .

59

Figure 39. Chert nodules in mi c r i t i c sediment. The nodules appear to have replaced burrow f i l l s .

PETROLEUM POTENTIAL

The regional petroleum p o te n tia l o f the Pedregosa Basin has been

previously examined by Greenwood e t a l . (1977) and Thompson e t a l .

(1978 ). While the Lower Cretaceous is considered a p o te n tia l hydro­

carbon producer, based on i ts l i th o lo g ic s im ila r it ie s to tim e-eq u iva len t

productive un its in the G ulf Coast, i t is ranked below several Paleozoic

horizons as an exp lo ra tio n o b je c tiv e . Thompson e t a l . (1978) consider

the limestones o f the Mural and c o rre la t iv e U-Bar Formation as the most

prospective Lower Cretaceous exp lo ra tion o b je c tiv e . They l i s t both

source rock and re s e rv o ir q u a lity as f a i r , considering patch reefs

w ith in the formations as the best re s e rv io r prospects. They note the

small s ize o f the patch reefs ( d i f f i c u l t d r i l l in g ta rg e ts ) and lack o f

observed good p erm eab ility in the formations as possib le drawbacks.

This study o f Bisbee Group carbonates in the Guadalupe Canyon area

has produced complementary observations. In southeastern A rizona, the

upper Mural Limestone represents deposition on a low energy s h e lf . The

m a jo rity o f sediments are m ic r i t ic w ith l i t t l e or no p o ro s ity . Even

pore space w ith in the scattered c o r a l-a lg a l- r u d is t patch reefs is

commonly m ic r ite f i l l e d (Grocock, 1975; S co tt; 1979). Primary p o ro s ity

may have preserved in the large reef-banks marking the E arly Cretaceous

s h e lf edge in Sonora, Mexico, where higher energy conditions capable o f

winnowing lime mud l ik e ly e x is ted . Localized o o l i t ic shoals on the

s h e lf , as described by Scott (1 9 7 9 ), represent possib le re s e rv o irs ,

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61

but th e ir d is tr ib u tio n w i l l be d i f f i c u l t to p re d ic t , and those observed

a t the outcrop have been t ig h t ly cemented.

In the Guadalupe Canyon a rea , the best p o te n tia l carbonate

reservo irs e x is t w ith in the mixed c la s t ic and carbonate section in the

upper portion o f the M o rita . P e lle t - r ic h zones and o o l i t i c , t id a l sand

bodies, although now t ig h t ly cemented, contained e x c e lle n t prim ary

in te rp a r t ic le p o ro s ity . In a d d itio n , the presence o f o r ig in a l evaporite

m inerals in lim estones, and p a r t ia l ly dolom itized lim estones, presented

e x c e lle n t s o lu b i l i t y contrasts fo r la te r secondary p o ro s ity development

due to p a r t ia l leaching by m eteoric w ater. Only a s l ig h t sea -leve l drop

would have been required to s u b a e r ia lly expose these very shallow

subtidal to supra tida l sediments. Although vuggy p o ro s ity was observed

in one very th in lim estone bed due to the probable d isso lu tio n o f

evaporite m in era ls , the major product o f freshw ater diagenesis in the

carbonates was t ig h t cementation w ith coarse, equant, mosaic sparry

c a lc ite . Freshwater d iagenetic processes such as leaching and non­

cementation may have a ffe c te d adjacent areas w ith s im ila r l ith o lo g ie s .

I f so, the presence o f e x c e lle n t prim ary p o ro s ity sediments and the

p o te n tia l fo r secondary p o ro s ity development in others due to inherent

s o lu b i l i t y co n tras ts , could have led to the development o f e x c e lle n t

re s e rv o irs . The sandstones observed in the section were a l l t ig h t .

Pressure so lu tion ing and cementation by quartz overgrowths destroyed any

p o ro s ity .

Due to the g e n e ra lly poor exposure in the study a rea , the lower

member o f the Mural was not a major research goal. Observations in the

62

f ie ld noted dom inantly low p o ro s ity , m ie n *tic carbonates. Abundant

sandstones are present and represent possib le re s e rv o ir ta rg e ts , but i f

th e ir d iagenetic h is to ry is s im ila r to the sandstones in the mixed

c la s t ic and carbonate section w ith in the underlying M orita Formation,

l i t t l e or no p o ro s ity is expected to be found.

Greenwood e t a l . (1977) mention the dark lim estones o f the U-

Bar Formation and equ ivalents as possib le petroleum source rocks.

Z e lle r (1965) considered the oyster lim estone member o f the U-Bar

Formation in the Big Hatchet Mountains as a possib le petroleum source

bed. W ithin th is member he noted a strong p e tro life ro u s odor from many

fre s h ly broken lim estone and sandstone beds, and a natura l gas odor

w hile digging in a shale bed. He also observed a drop o f l iq u id

petroleum flow from an in te rn a l c a v ity in a fre s h ly broken pelecypod

sh e ll from a dense lim estone. Many o f the carbonate beds in the study

area are f a i r l y dark and release a hydrogen s u lf id e odor from fresh

breaks. Although a q u a lita t iv e c o rre la tio n ex is ts between carbonate

co lor and organic m atter - ferrous s u lf id e con ten t, chemical analyses

of dark, fe t id Mural and M orita carbonates and equ ivalents fo r to ta l

organic carbon content should be made to evaluate th e ir tru e source rock

p o te n tia l. Based on th is param eter, southwestern New Mexico ra th e r than

southeastern Arizona appears to be the most prospective d r i l l in g area

fo r Lower Cretaceous petroleum production. The carbonate section is

th ic k e r , source beds commonly being o f marine o r ig in , and the most

promising observations fo r the presence o f a possib le source bed have

been noted from th is area ( Z e l le r , 1965).

SUMMARY

Lower Cretaceous Bisbee Group outcrops in the Guadalupe Canyon

area o f extreme southeastern Cochise County, A rizona, consist o f the

M orita Formation and Mural Limestone. A mixed c la s t ic and carbonate

section in the upper portion o f the M orita Formation marks the f i r s t

s ig n if ic a n t Lower Cretaceous carbonate deposition in the study area .

Sediments were deposited in a very shallow sub tidal to sup ra tida l en­

vironment in an a rid c lim a tic s e tt in g . P eriod ic m eteoric w ater in flu x

in to the environment induced local sch izohaline conditions and asso­

c ia ted diagenesis of the carbonate sediments.

While the upper Mural Limestone is equ iva len t in thickness to

the type sec tio n , the lower Mural is tw ice as th ic k . The two members

are interbedded in the study area , w ith a th in s tr in g e r o f upper Mural

located w ith in the lower M ural. Warzeski (1979) observed a rap id

th ickening o f the upper Mural from the type-sec tion south in to northern

Sonora, Mexico. I t is very l ik e ly th a t the two tongues o f upper Mural

in the study area also thicken to the south, ev e n tu a lly coalescing in to a

sing le u n it . This im plies th a t the two members o f the Mural Limestone

in te r f in g e r over great (10s o f k ilo m eters) d istances. The upper Mural

represents the major period o f Lower Cretceous carbonate deposition in

the area . M ic r i t ic sediments were deposited in a g e n e ra lly low energy,

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64

lo c a lly s l ig h t ly reducing, s h e lf environment w ith scattered c o r a l-a lg a l-

ru d is t patch reefs and associated shoals.

The mixed c la s t ic and carbonate section in the upper portion o f

the M orita Formation contains the best p o te n tia l petroleum re s e rv o irs .

Although l i t t l e p o ro s ity was observed in the carbonates a t the outcrop,

the occurrence o f numerous beds w ith (1 ) o r ig in a l e x c e lle n t prim ary

in te rp a r t ic le p o ro s ity or (2 ) inherent s o lu b i l i t y contrasts fo r the

possible development o f secondary p o ro s ity , make th is s tra tig ra p h ic

in te rv a l the best prospect in the Lower Cretaceous in the study area.

APPENDIX A

MEASURED STRATIGRAPHIC SECTIONS

Units are numbered in ascending order up through each sec tio n , thus u n it one represents the base o f each sec tion .

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66

Section 1: Mixed e la s tic s and Carbonates, M orita FormationSection measured up the w est-fac ing ridge in the

SWkNEk sec. 13, T. 24 S ., R. 31 E . , Guadalupe Canyon Quadrangle, Cochise County, Arizona

U nit Th ickness-feetNo.____________________D escription __________________________ U n it Cumulative

1 Dolomite (sandy d o lo m ic r ite ); ye llow ish gray 1 .5 1 .5(5Y 7 /2 ) , weathering pale ye llow ish orange (10YR 8 /6 ) to grayish orange (10YR 7 /4 ) ; abundant very f in e sand-sized terrigenous quartz; sample I IA .

Covered 4 .6 6 .1

2 Dolom itic limestone (sandy to very sandy 0 .9 7 .0dolom itized b io s p a r ite ) ; medium gray (N5) andpale yellow ish brown (10YR 6 /2 ) , weathering l ig h t gray (N7) and pale yellow ish orange (10YR 8 /6 ) ; much o f the fo s s il debris replaced by sparry c a lc i te ; u n it contains a few m ic r i t ic in tra c la s ts ; u n it shows a crude horizon ta l s t r a t i f ic a t io n on a scale o f 1 .0 -2 .0 cm; the very sandy top 5 .0 cm has p lanar laminae on a scale of 1 .0 -2 .0 mm; sample H Z .

3 Dolom itic limestone (very sandy, d o lo m itized , 2 .0 9 .0fo s s ilife ro u s oosparite grading to sandy,do lom itized , fo s s ili fe ro u s , in tr a c la s t ic oosparite to sandy, do lo m itized , f o s s i l i f ­erous, o o l i t ic in t r a s p a r ite ) ; dark gray (N3) and l ig h t brownish gray (SYR 6 /1 ) , weathering pale yellow ish orange (10YR 8 /6 ) and l ig h t gray (N7) to medium l ig h t gray (N 6); ooids average 0 .2 5 -0 .5 0 mm in diam eter; terrigenous quartz grains have la rge euhedral over­growths; the top 5 .0 -7 .5 cm has ex c e p tio n a lly w ell developed ooids (1 .0 mm average diam eter) and a m icro-scour surface a t the base o f the zone; the bottom 20 cm is a very sandy, do lom itized , fo s s ilife ro u s oosparite - the zone contains abundant very f in e sand­sized terrigenous quartz grains lacking overgrowths; the zone has trough c ro s s -s tra ta w ith sets averaging 2 .5 - 5 .0 cm in th ickness; samples I IB , IIB S , LH-1.

67

4 Limestone (m ic r ite -s t r o m a to li t ic ) ; weathers 0 .2pale ye llow ish orange (10YR 8 /6 ) and mediumlig h t gray (N 6 ); u n it is o f v a r ia b le th ic k ­ness; u n it is poorly exposed; sample I IC .

5 Dolom itic limestone (do lom itized p e lm ic rite to 1 .0dolom itized p e ls p a r ite ); medium dark gray(N 4), weathering l ig h t gray (N7) and grayish orange (10YR 7 /4 ) to dark yellow ish orange (10YR 6 /6 ) ; u n it has a m ottled appearance; u n it poorly exposed; sample IIC A .

6 Dolom itic limestone (do lom itized p e lm ic rite to 0 .4dolom itized p e ls p a r ite ); dark gray (N 3); black( N l ) , and dark yellow ish orange (10YR 6 /6 ) , weathering l ig h t gray (N 7 ), black ( N l ) , and dark yellow ish orange (10YR 6 /6 ) ; lo c a l ly u n it contains in d is t in c t c ro s s -s tra ta w ith sets averaging 1 .0 -2 .0 cm in th ickness; u n it contains small patches o f b lack , coarse cal c ite - possib ly a replacement o f former evaporite m inera ls ; u n it is poorly exposed; sample IIC B .

7 Dolom itic lim estone (do lom itized p e lm ic rite to 0 .7 dolom itized p e ls p a r i te ) ; dark gray (N 3 ), weathering medium l ig h t gray and paleyellow ish orange (10YR 8 /6 ) ; s l ig h t hydrogen s u lf id e odor on fresh breaks; u n it has a m ottled appearance; u n it is very poorly exposed; sample I ID .

Covered 7 .5

8 Dolomite ( f in e ly c r y s ta ll in e p e l le t d o lo m ite ); 0 .8medium gray (N5) to l ig h t brownish gray(SYR 6 /1 ) , weathering medium gray (N5) and grayish orange (10YR 7 /4 ) ; u n it contains small lenses ric h in f in e sand-sized terrigenous quartz g ra ins; lo c a l ly the u n it constains wispy laminae and trough c ro s s -s tra ta ; the top o f the u n it has been reworked and in te r f in g e rs w ith the overly ing u n it ; the u n it is very poorly exposed; sample H E .

9 Dolom itic limestone (very sandy, d o lo m itized , 1 .2o o s p a rite ); medium dark gray (N 4 ), weatheringl ig h t gray (N7) to pale ye llow ish orange (10YR 8 /6 ) ; u n it contains abundant very f in e

9 .2

10.2

10.6

11.3

18.8

19.6

20.8

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sand-sized terrigenous q u a rtz ; u n it contains trough c ro s s -s tra ta w ith sets averaging 3 .0 -4 .0 cm in th ickness; basal portion o f u n it contains in tra c la s ts derived from underlying u n it ; ooids average 0 .2 0 -0 .2 5 mm in diam eter; sample I IP .

Covered 8 .6

10 Dolom itic limestone (d o lo m itized , 0 .3fo s s ilife ro u s oosparite to dolom itized b io m ic r ite -b io m ic ru d ite ); dark gray (N3) tograyish black (N 2 ), weathering medium dark gray (N4) and dark yellow ish orange (10YR 6 /6 ) ; s l ig h t hydrogen s u lf id e odor on fresh breaks; she ll debris averages 1 .0 -3 .0 mm in s iz e ; u n it shows a crude horizon ta l s t r a t i f ic a t io n ; u n it is very poorly exposed; sample IIGB.

11 Dolom itic limestone (do lom itized b io m ic r ite - 0 .4b iom icrud ite to dolom itized b io s p a rite - b io s p a rru d ite ); dark gray (N3) and darkyellow ish orange (10YR 6 /6 ) , weathering medium dark gray (N4) and dark yellow ish orange (10YR 6 /6 ) ; she ll debris averages 1 .0 -3 .0 mm in s iz e ; top bedding plane contains small patches o f lim o n ite a f te r p y r ite ; in some instances the lim o n ite a f te r p y r ite has replaced she ll m a te r ia l; u n it isvery poorly exposed; sample IIGA.

Covered 5 .3

12 Limestone ( fo s s il i fe ro u s o o s p a rite ); medium 2.1gray (N5) to dark gray (N 3 ), weathering mediumgray (N5) and pale ye llow ish orange (10YR 8 /6 ) ; ooids average 0 .35 mm in diam eter and are p r e fe r e n t ia l ly s i l i c i f i e d over the m atrix ; u n it is trough c ro s s -s tra t if ie d w ith sets up to 16 mm in th ickness; samples IIH ,L H -2 .

Covered 11 .4

13 Dolom itic limestone (sandy d o lo m icrite and 0 .8sandy, do lom itized , sparse b io m ic r ite );moderate ye llow ish brown (10YR 5 /4 ) , weathering dark yellow ish orange (10YR 6 /6 ) ; u n it has a m ottled appearance w ith d isc re te

29.4

29.7

30.1

35.4

37.5

48.9

49.7

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burrow f i l l s ; abundant very f in e sand-sized terrigenous quartz g ra in s ; m a jo r ity o f u n it is d o lo m ic rite , the burrow f i l l s are dolom itized sparse b io m ic rite ; u n it poorly exposed; sample I I J .

14 Dolom itic lim estone (do lom itized b io m ic r ite ); 1 .3 51.0dark gray (N3) and dark yellow ish orange (10YR 6 /6 ) , weathering grayish orange (10YR 7 /4 ) to pale ye llow ish orange (10YR 8 /6 ) ; fo s s il debris averages 0 .5 -1 .5 mm in s iz e , w ith most replaced by sparry cal c i te ; percentage o f fo s s il debris increases up through u n it ; lower h a lf o f u n it has rare m ic r i t ic burrow f i l l s - mixing o f sediment from u n it 1 3 .; upper h a lf o f u n it contains a few large pelecypod she ll fragments o f black c a lc ite commonly p a r t ia l ly s i l i c i f i e d ; sample U K .

Covered 7 .0 58.0

15

16

17

Dolom itic limestone (sandy, do lo m itized , 0 .9 58.9fo s s ilife ro u s m ic r ite grading to sandy,dolom itized b io m ic r ite ); l ig h t gray (N7) topale ye llow ish brown (10YR 6 /2 ) , weatheringpale ye llow ish orange (10YR 8 /6 ) to darkyellow ish orange (10YR 6 /6 ) ; much o f the she lldebris replaced by sparry c a lc i te ; abundantvery f in e sand-sized terrigenous quartz g ra in s;u n it poorly exposed; sample IIW .

Dolom itic limestone (d o lo m itized , o o l i t ic 0 .8 59.7b io s p a rite to do lom itized , fo s s ilife ro u so o s p a rite ); o liv e gray (5Y 4 /1 ) , weatheringlig h t gray (N7) and pale ye llow ish orange(10YR 8 /6 ) to dark ye llow ish orange (10YR 6 /6 ) ;s lig h t hydrogen s u lf id e odor on fresh breaks;fo s s il debris averages 0 .5 -1 .5 mm in s iz e ;lo c a lly la rge pelecypod sh e ll fragments -occas iona lly replaced by sparry c a lc iteor p a r t ia l ly s i l i c i f i e d ; u n it very poorlyexposed; sample I IV .

Covered 9 .7 69.4

Limestone (p e l le t i fe r o u s , fo s s ilife ro u s 1 .3 70.7oom icrite to p e l le t i fe r o u s , fo s s ilife ro u s o o s p a rite ); dark gray (N 3 ), weathering l ig h t gray (N7) and pale ye llow ish orange

70

18

19

(10YR 8 /6 ) to dark ye llow ish orange (10YR 6 /6 ) ; f a i r hydrogen s u lfid e odor on fresh breaks; fo s s il debris averages 0 .5 -1 .5 mm in s iz e ; ooids average 0.50 mm in diam eter; sample I IX .

Covered 1.5 72.2

Limestoneoom icriteoosparite )

(p e l le t i fe r o u s , fo s s ilife ro u s to p e l le t i fe ro u s , fo s s ilife ro u s ; same as u n it 17.

0 .9 73.1

Covered 2 .0 75.1

Limestone (fo s s ilife ro u s o o s p a r ite ); dark gray 2 .0 77.1(N3) to brownish gray (SYR 4 /1 ) , weathering l ig h t gray (N7) and pale yellow ish orange (10YR 8 /6 ) to dark ye llow ish orange (10YR 6 /6 ) ; s lig h t hydrogen s u lf id e odor on fresh breaks; ooids average 0 .30 mm in diam eter and are pre­f e r e n t ia l ly s i l i c i f i e d over the m atrix ; small abraded pelecypod fragments are lo c a l ly present and commonly replaced by sparry cal c i t e ; u n it is lo c a lly trough c ro s s -s tra t if ie d w ith sets averaging 2 cm in th ickness; samples I IY , LH-3.

Covered 3 .5 80.6

20 Limestone ( fo s s ilife ro u s oosparite to o o l i t ic 3 .8 84.4b io p e ls p a r ite ); dark gray (N3) to brownishgray (SYR 4 /1 ) , weathering pale yellow ish orange (10YR 8 /6 ) and l ig h t gray (N7) to medium gray (N 5); strong hydrogen s u lf id e odor on fresh breaks; ooids average 0 .50 mm in s ize ; many she ll fragments in lower h a lf of u n it are replaced by s i l i c a , sometimes in the form o f euhedral quartz c ry s ta ls ; lower h a lf o f u n it contains s ty lo ! i te s w ith s i l ic a concentrated along seams; u n it has a m ottled appearance; top 0 .5 fe e t is an o o l i t ic b iosparrud ite w ith numerous pelecypod she ll fragments - 0 .5 -1 .0 cm in s ize and commonly p a r t ia l ly s i l ic i f ie d ; sample H U .

21 Limestone ( fo s s il i fe ro u s oosparite to 5 .0 89 .4b io s p a r ite -b io m ic r ite ) ; dark gray (N3) tobrownish gray (SYR 4 /1 ) , weathering l ig h t gray (N7) and brownish gray (SYR 4 /1 ) ; strong hydrogen s u lf id e odor on fresh breaks; ooids average 0 .50 mm in diam eter; lo c a l ly .

abundant, la rge pelecypod sh e ll fragments are present - commonly s i l i c i f i e d in the form o f large euhedral quartz c ry s ta ls ; the u n it contains lo c a l, in d is t in c t c ro s s -s tra ta ; basal 1 .5 fe e t o f u n it is an o o l i t ic b iosparrud ite in which the she ll debris is commonly s i l i c i f i e d ; samples IIT A , IIT B , IIT C , LH-4.

Limestone ( o o l i t ic b io s p a rite to o o l i t ic b io m ic r ite ); dark gray (N3) to dusky red (SYR 3 /4 ) , weathering l ig h t gray (N7) and l ig h t brownish gray (5YR 6 /1 ) ; strong hydrogen s u lf id e odor on fresh breaks; ooids average 0 .5 -1 .0 mm in diam eter w ith the fo s s il debris being s l ig h t ly la rg e r ; occasional, la rg e (0 .5 -1 .0 cm average) she ll fragments present, including echinoids and h igh-sp ired gastropods; the she ll fragments are commonly s i l i c i f i e d in the form o f la rg e , euhedral quartz c ry s ta ls ; sample I IS .

Covered

Limestone ( o o l i t ic b io m ic rite to o o l i t ic b io m ic ru d ite ); dark gray (N3) to brownish gray (SYR 4 /1 ) , weathering l ig h t gray (N 7 ), dark gray (N 3 ), and moderate orange pink (10R 7 /4 ) ; strong hydrogen s u lf id e odor on fresh breaks; u n it has m ottled apperance; sample H O .

Dolom itic lim estone (d o lo m itized , fo s s ilife ro u s o o s p a rite ); dark gray (N 3 ), weathering l ig h t gray (N 7); strong hydrogen s u lf id e odor on fresh breaks; ooids average 0 .5 -1 .0 mm in diam eter; u n it contains ir re g u la r zones o f s i l i c i f i c a t io n which appear to be burrow f i l l s ; samples I IP 1 , I IP 2 .

Limestone (p e lle t ife ro u s b io s p a r ite ) ; dark gray (N3) to brownish gray (SYR 4 /1 ) , weathering l ig h t gray (N 7); f a i r hydrogen s u lf id e odor on fresh breaks; sh e ll debris is crude ly , h o r iz o n ta lly lam inated; top o f underlying u n it m o ttled , contain ing in f i l l in g s o f sediment from th is u n it ; poorly exposed; sample H Q .

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26 Limestone (p e lle t ife ro u s b io m ic rite to 0 .7 101.0p e lle t ife ro u s b io m ic ru d ite ); dark gray (N3) to grayish black (N 2 ), weathering l ig h t gray (N 7 ), dark gray (N 3 ), and dark yellow ish orange (10YR 6 /6 ) ; strong hydrogen s u lf id e odor on fresh breaks; u n it is d is t in c t ly m ottled; u n it contains rare abraded she ll debris and abundant s i l i c i f i e d worm tubes; poorly exposed; sample H R .

END OF SECTION - section ends a t top o f west facing ridge where a bedding plane f a u l t is encountered and cover on top o f ridge increases g re a t ly .

73

Section 2: Massive Carbonates, Lower Mural LimestoneSection measured up the south-facing c liffs in the

SE% SEW sec. 17, T. 24 S., R. 32 E., GuadalupeCanyon Quadrangle, Cochise County, Arizona

U nit Th ickness-feetNo.___________________D escrip tion___________________________ U n it Cumulative

1 Limestone ( fo s s il i fe ro u s in tr a m ic r ite ) ; medium 7 .8 7 .8gray (N 5 ), weathering l ig h t gray (N 7); in t r a ­c las ts and fo s s il debris range from 0 .2 5 -2 .0 mm in s iz e ; numerous hem ispheroidal, cero id cora ls (A c tin as trea?) and branching, ceroid c o ra ls ,(commonly densely bored and r e c r y s ta l l iz e d ) ; lo c a l ly , 2 .0 -3 .0 cm, p a r t ia l ly s i l i c i f i e d oyster she ll fragments; sample 1H, I I .

2. Limestone (b io m ic r ite and fo s s ilife ro u s in t r a - 0 .2 -0 .8 8 .6 m ic r i t e ) ; weathers medium gray (N5) and l ig h t gray (N 7); m ottled appearance; v a ria b le th ic k ­ness w ith lower-bounding surface o f reworking; weathers more re a d ily than adjacent u n its , forming shallow caves.

3. Limestone ( fo s s il i fe ro u s in t r a m ic r ite ) ; same 3 .5 12.1as u n it 1.

4. Limestone (b io m ic r ite and fo s s ilife ro u s in t r a - 0 -1 .6 m ic r i te ) ; weathers l ig h t gray (N7) and grayish orange (10YR 7 /4 ) , w ith m ottled appearance due to te x tu ra l v a r ia t io n ; the grayish orange­weathering zones ( fo s s il i fe ro u s in tra m ic r ite ) contain coarser (averag e -s ize : 1 .0 -2 .0 mm) and more t ig h t ly packed allochems than the l ig h t gray-weathering zones (b io m ic r ite ) (average allochem s ize : 0 .2 5 -0 .5 0 mm); u n it is o f v a r i ­able thickness w ith lower-bounding surface o f reworking; weathers more r e a d ily than ad­jacen t u n its , forming shallow caves.

* Where u n it 4 is th ic k , u n it 5 is not present; where u n it 4 is th in or absent, u n it 5 is th ic k .

5 Limestone ( fo s s il i fe ro u s in tr a m ic r ite ) ; same 0 -2 .2 14.3as u n it 1.

6. Limestone (o n c o lit ic , in t r a c la s t ic , oyster 1 .2b io m ic ru d ite ); medium gray (N 5 ), weatheringlig h t gray (N 7); in tra c la s ts , o n c o lite s , and f in e she ll debris range from 0 .2 5 -2 .0 mm in s iz e ; abundant, 1 .0 -3 .0 cm oyster she ll fra g ­ments o f black c a lc ite (o fte n p a r t ia l ly s i l i c i f i e d ) ; possib le s lig h t scour surface along bottom o f u n it ; sample IF .

7. Limestone ( fo s s il ife ro u s , o n c o lit ic , in t r a - 12.2m ic rite to fo s s il ife ro u s , in t r a c la s t ic ,o n c o lit ic m ic r i te ) ; medium gray (N5) to medium dark gray (N 4 ), weathering l ig h t gray (N 7); very s lig h t hydrogen s u lf id e odor on fresh breaks; allochems range from 0 .5 -2 .0 mm in s iz e ; rare oyster she ll fragments (as described in u n it 6 ) ; from 4 .0 -6 .5 fe e t above the base o f the u n it is a zone of exceptional o n c o lite development - the average o n c o lite diameter is 3 .0 -4 .0 mm, w ith some reaching8 .0 mm; in the top 5 .5 fe e t o f the u n it , two 1 .0 fo o t th ic k zones contain rare branching and hemispheroidal corals (as described in u n it 1 ) ; samples ID , 1C.

8. Limestone (o n c o lit ic , in t r a c la s t ic , oyster 1 .0b io m ic ru d ite ); medium dark gray (N 4 ),weathering l ig h t gray (N7) and grayish pink (5R 8 /2 ) ; s lig h t hydrogen s u lf id e odor on fresh breaks; in tra c la s ts , o n c o lite s , and f in e fo s s il debris average 1 .0 -1 .5 mm in s iz e ; abundant oyster she ll fragments (as de­scribed in u n it 6 ) .

9. Limestone ( fo s s il i fe r o u s , o n c o lit ic , in t r a - 6 .1m ic rite to fo s s ili fe ro u s , in t r a c la s t ic ,o n c o lit ic m ic r i te ) ; medium dark gray (N 4 ), weathering l ig h t gray (N7) and grayish pink (5R 8 /2 ) ; allochems range from 0 .5 -2 .0 mm in s iz e ; ra re oyster she ll fragments (as described in u n it 6 ) ; onco lites ex c e p tio n a lly w ell de­veloped in the upper 1 .0 fo o t o f u n it w ith an average diam eter o f 2 .5 -3 .0 mm, and some reaching 6 .0 mm; sample IB , 1A.

END OF SECTION - section ends a t top o f south-facing c l i f f s .

74

15.5

27.7

28.7

34.8

75

Section 3: Massive Carbonates, Lower Mural LimestoneSection measured up the south-facing c liffs in the

SW%SW% sec. 16, T. 24 S ., R. 32 E., GuadalupeCanyon Quadrangle, Cochise County, Arizona

U nit Th ickness-feetNo.______________________ D escrip tion_________________________ U n it Cumulative

1 Limestone ( fo s s il i fe ro u s in t r a m ic r ite ) ; 7 .8 7 .8medium dark gray (N4) to brownish gray (SYR 4 /1 ) , weathering l ig h t gray (N 7); s lig h t hydrogen s u lf id e odor on fresh breaks; in tra c la s ts and fo s s il debris range from 0 .2 5 -2 .0 mm in s iz e ; numerous hemi- sphero idal, ceroid corals ( A c tin astrea? ) and branching, ceroid corals - commonly, densely bored and re c ry s ta ll iz e d ; ra re 2 .0 -3 .0 cm p a r t ia l ly s i l i c i f i e d oyster she ll fragments; from 1 .5 fe e t to 2 .0 fe e t above the base, u n it weathers re a d ily , being commonly recessed - may be analogous to u n it 2.

2. Limestone (b io m ic r ite and fo s s ilife ro u s in t r a - 0 .3 8 .1m ic r i t e ) ; weather l ig h t gray (N7) and paleyellow ish brown (10YR 6 /2 ) to pale yellow ish orange (10YR 8 /6 ) w ith m ottled appearance; v a r i ­able thickness w ith lower-bounding surface o f reworking; u n it weathers more re a d ily than adjacent u n its , being recessed along c l i f f face.

3. Limestone ( fo s s il i fe ro u s in tr a m ic r ite ) ; same 3 .0 11.1as u n it 1.

4. Limestone (b io m ic r ite and fo s s ilife ro u s in t r a - 1 .0 12.1m ic r i te ) ; same as u n it 2 , although not as g re a t­ly recessed along c l i f f face ; m ottled appearance due to te x tu ra l v a r ia t io n ; the pale ye llow ish brown to pale ye llow ish orange - weathering zones ( fo s s ilife ro u s in tra m ic r ite ) contain coarser,(averag e-s ize : 1 .0 -2 .0 mm) and more t ig h t ly packed allochems than the l ig h t gray-w eathering zones (b io m ic r ite ) (average allochem s ize : 0 .25 -0 .5 0 mm); sample BH-A.

76

5. Limestone (o n c o l it ic , in t r a c la s t ic , oyster b io - 2 .3 14.4m ic ru d ite ); medium dark gray (N4) to dark gray (N 3 ), weathering l ig h t gray (N 7); f a i r hydrogen s u lf id e odor on fresh breaks; abundant, la rge oyster she ll fragments o f black c a lc ite - o ften p a r t ia l ly s i l i c i f i e d ; ra re c o ra l, fe n e s tra te bryo- zoan, and h igh -sp ired gastropod fragments; some she ll m ateria l replaced by sparry c a lc i te ; in tra c la s ts and f in e fo s s il debris average 0 .5 -1 .0 mm in s iz e ; a few o n co lites occur, e s p e c ia lly in the upper portion o f the u n it .

6. Limestone ( fo s s il i fe r o u s , o n c o lit ic , in t r a - 11.3 25.7m ic r ite to fo s s ili fe ro u s , in t r a c la s t ic , o n c o lit ic m ic r ite ) medium dark gray (N4) to brownish gray (SYR 4 /1 ) , weathering l ig h t gray (N 7); s lig h t hydrogen s u lf id e odor on fresh breaks; allochems average 1 .0 -3 .0 mm in s iz e ; lo c a l ly , f a i r amount o f ru d ite -s iz e d fo s s il m a te r ia l, including h igh -sp ired gastropods and oyster she ll fragments (as described in u n it 5 ) ; some fo s s il m ateria l replaced by sparry c a lc i te ; between 3 .0 and5 .0 fe e t from the bottom o f the u n it is a zone o f e s p e c ia lly good o n c o lite development; the onco lites average 3 .0 -3 .5 mm in diam eter w ith some reaching 6 .0 -7 .0 mm.

END OF SECTION - section ends a t top o f south­facing c l i f f s .

77

U nit Th ickn ess-feetNo.____________________ D escription___________________________ U n it Cumulative

Section 4: Massive Carbonates, Lower Mural LimestoneSection measured up the south-facing c liffs in the

SWkSEk sec. 17, T. 24 S., R. 32 E., GuadalupeCanyon Quadrangle, Cochise County, Arizona

1 Limestone ( fo s s il i fe ro u s in tr a m ic r ite ) ; medium 5 .2 dark gray (N4) to dark gray (N 3 ), weatheringl ig h t gray (N 7); very s lig h t hydrogen s u lf id e odor on fresh breaks; in tra c la s ts and fo s s il debris range from 0 .2 5 -3 .0 mm in s iz e ; numerous hem ispheroidal, cero id corals ( A c tin a s tre a ?) and branching, ceroid corals - commonly densely bored and re c ry s ta ll iz e d ; ra re , la rg e , oyster she ll fragments o f black cal c i te - o ften p a r t ia l ly s i l i c i f i e d .

2 Limestone (b io m ic r ite and fo s s ilife ro u s in t r a ­m ic r i te ) ; weathers l ig h t gray (N7) and pale yellow ish brown (10YR 6 /2 ) , w ith m ottled appear­ance; u n it is o f v a ria b le th ickness, w ith lower-bounding surface o f reworking; u n it weathers more re a d ily than adjacent u n its , being recessed along c l i f f face .

3. Limestone ( fo s s ilife ro u s in tr a m ic r ite ) ; same 3 .7as u n it 1.

4. Limestone (b io m ic rite and fo s s ilife ro u s in t r a - 0 .8 m ic r i t e ) ; same as u n it 2; m ottled appearanceis due to te x tu ra l v a r ia t io n ; the pale ye llow ­ish brown-weathering zones ( fo s s ilife ro u s in t r a ­m ic r ite ) contain coarser (averag e -s ize : 1 .0 -2 .0 mm) and more t ig h t ly packed allochems than the l ig h t gray-weathering zones (b io m ic r ite )(average allochem s ize 0 .2 5 -0 .5 0 mm); sample BH-2.

5. Limestone (o n c o lit ic , in t r a c la s t ic , oyster b io - 2 .0 m ic ru d ite ); dark gray (N3) to brownish gray(SYR 4 /1 ) , weathering l ig h t gray (N 7); s l ig h t hydrogen s u lf id e odor on fresh breaks; abundant, la rge oyster she ll fragments o f black c a lc ite - o ften p a r t ia l ly s i l i c i f i e d ; ra re coral fra g ­ments - densely bored and commonly re c ry s ta ll iz e d

5 .2

9.1

9 .9

11.9

78

and fe n e s tra te bryozoan fragments; some fo s s il m ateria l replaced by sparry c a lc i te ; in t r a ­c la s ts and f in e fo s s il debris average 0 .5 -1 .0 mm in s iz e ; a few onco lites occur, e s p e c ia lly in the upper portion o f the u n it .

6. Limestone ( fo s s il i fe r o u s , o n c o lit ic , in t r a - 11.5 23.4m ic rite to fo s s ili fe ro u s , in t r a c la s t ic ,o n c o lit ic m ic r i te ) ; medium dark gray (N4) to l ig h t brownish gray (SYR 6 /1 ) , weathering l ig h t gray (N 7); a l l allochems range from 1 .0 -3 .0 mm in s iz e ; ra re oyster she ll fra g ­ments (as described in u n it 5 ) ; from 3 .5 to6 .0 fe e t above the base o f the u n it , is a zone o f exceptional o n c o lite development; the average o n c o lite diam eter is 4 .0 - 4 .5 mm, w ith some reaching 8 .0 -9 .0 mm; the top 1 .0 fo o t o f the u n it contains rare hemispheroidal and branching cora ls (as described in u n it 1 ) .

7. Limestone (o n c o l it ic , in t r a c la s t ic , oyster 1 .2 24.6b io m ie ru d ite ); medium dark gray (N 4 ), todark gray (N 3 ), weathering l ig h t gray (N7) and grayish pink (5R 8 /2 ) ; s lig h t hydrogen s u lf id e odor on fresh breaks; in tra c la s ts o n c o lite s , and f in e fo s s il debris average 0 .5 -1 .5 mm in s iz e ; abundant oyster she ll fragments (as described in u n it 5 ) ; u n it is s l ig h t ly recessed along c l i f f face .

8. Limestone ( fo s s il i fe r o u s , o n c o lit ic , in t r a - 4 .2 28.8m ic r ite to fo s s ili fe ro u s , in t r a c la s t ic ,o n c o lit ic m ic r i te ) ; medium dark gray (N 4 ), weathering l ig h t gray (N 7); s l ig h t hydrogen s u lf id e odor on fresh breaks; allochems range from 0 .5 -2 .0 mm in s iz e ; ra re oyster she ll fragments (as described in u n it 5) and coral fragments (as described in u n it 1 ) .

END OF SECTION - section ends a t top o f south-facing c l i f f s .

79

Section 5: Massive Carbonates, Lower Mural LimestoneSection measured up the south-facing c liffs inthe NW% SWk sec. 5, T. 24 S., R. 29 E., College

Peaks Quadrangle, Cochise County, Arizona

U nit T h ickn ess-feetNo._____________________ D escription__________________________ U n it Cumulative

1 Limestone (sandy, fo s s ilife ro u s in tra m ic r ite 4 .0grading to sandy, d o lo m itic , in tr a c la s t icoyster b io m ic ru d ite ); medium dark gray (N 4 ), weathering l ig h t gray (N7) and grayish orange (10YR 7 /4 ) to dark ye llow ish orange (10YR 6 /6 ) ; s lig h t hydrogen s u lf id e odor on fresh breaks; in tra c la s ts and she ll d eb ris , includ ing bryozoan and h igh -sp ired gastropod fragments, average 1 .0 mm in s iz e ; some shell debris replaced by sparry c a lc i te ; ra re o n c o lite s ; large oyster fragments o f black c a lc ite - commonly bored and p a r t ia l ly s i l i c i f i e d occur in the top 1 .0 fo o t; th is 1 .0 fo o t zone has a m ottled te x tu re due to p a r t ia l d o lo m itiza tio n o f the lim estone; sample PERI 1.

2 Limestone (sandy, fo s s il i fe r o u s , in t r a c la s t ic , 7 .5 o n c o lit ic m ic r i te ) ; medium dark gray (N 4 ), weathering l ig h t gray (N 7); s l ig h t hydrogens u lf id e odor on fresh breaks; onco lites average 2 .0 -3 .0 mm in diam eter w ith a few reaching 5 .0 -6 .0 mm; occasional la rge oyster fragments (as described in u n it 1 . ) ; sample PERI I I .

3 Limestone (sandy, in tr a c la s t ic oyster 3 .0b io m ic ru d ite ); medium dark gray (N4) tobrownish gray (SYR 4 /1 ) , weathering l ig h t gray (N 7); f a i r hydrogen s u lf id e odor on fresh breaks; in tra c la s ts and abraded she ll debris average 1 .0 mm in s iz e ; ra re o n c o lite s ; abundant la rge oyster fragments (as described in u n it 1 . ) ; ra re coral fragments - densely bored and re c ry s ta l l iz e d .

4 Limestone (sandy, fo s s life ro u s in tra m ic r ite to 4 .5 sandy, fo s s ili fe ro u s , o n c o lit ic in tr a m ic r ite ) ; medium dark gray (N4) to brownish gray

4 .0

11.5

14.5

19.0

(SYR 4 /1 ) , weathering l ig h t gray (N7) and lo c a l ly grayish orange (10YR 7 /4 ) ; f a i r hydrogen s u lf id e odor on fresh breaks; in tra c la s ts and she ll d eb ris , including bryozoan and h igh- spired gastropod fragm ents, average 1 .0 mm in s iz e ; some sh e ll debris replaced by sparry cal c i te ; occasional, la rg e oyster fragments (as described in u n it 1 . ) ; lo c a lly the u n it has a m ottled te x tu re due to the p a r t ia l d o lo m itiza tio n o f the lim estone; th ic k ly - bedded.

END OF SECTION - section ends a t top o f south-facing c l i f f s .

81

Section 6: Non-Reefal Upper Mural limestoneSection measured up the south-west facing c liffs

in the NW% SE% sec. 33, T. 23 S., R. 32 E.,Guadalupe Canyon Quadrangle, Cochise County, Arizona

U nit Th ickness-feetNo.__________________D escrip tion_______________ _____________U n it Cumulative

1 Limestone (sparse b io m ic r ite ); medium dark 3 .0 3 .0gray (N4) to brownish gray (SYR 4 /1 ) , weathering l ig h t gray (N 7); f a i r hydrogen s u lf id e odor on fresh breaks; very fra g ­mented fo s s il debris averaging 0 .5 -1 .0 mm in s ize - commonly s i l i c i f i e d or replaced by sparry cal c i te ; sample CINT-K.

Covered 2 .0 5 .0

2 Limestone (sparse b io m ic r ite ); same as u n it 1. 3 .1 8 .1

Covered 5 .2 13.3

3 Limestone (fo s s il ife ro u s m ic r ite grading w ith 29 .0 42.3sparse O rb ito lin a b io m ic rite to sparseO rb ito lin a b io m ic ru d ite ); o liv e gray (5Y 4 /1 ) to brownish gray (SYR 4 /1 ) , weathering l ig h t gray (N7) to very l ig h t gray (N 8); s l ig h t hydrogen s u lf id e odor on fresh breaks; ir re g u la r ly shaped ch ert nodules common in upper nine fe e t o f u n it are black (N l) weathering l ig h t brown (SYR 5 /6 ) ; s t y le l i t e s , often concentrated w ith lim o n ite /h e m a tite ; th ic k ly bedded; sample CINT-A.

Covered 7 .0 49 .3

4 Limestone (packed b io m ic rite grading to b io - 4 .5 53 .8m ic ru d ite ); medium gray (N5) and grayishorange (10YR 7 /4 ) , weathering l ig h t gray (N7) and pale yellow ish orange (10YR 8 /6 ) ; s l ig h t hydrogen s u lf id e odor on fresh breaks; fo s s il d eb ris , including O rb ito lin a , averages 0 .2 5 -0 .5 0 mm in s iz e ; s l ig h t m ottled appearance; top 1 .5 fe e t contains la rg e

(averag e-s ize : 1 .0 -2 .0 cm) sh e ll fra g ­ments Including h igh -sp ired gastropods and ribbed pelecypods; she ll m ateria l commonly s i l i c i f i e d , w ith l iv in g c a v it ie s f i l l e d w ith a mosaic sparry c a lc i te ; sample CINT-L.

Covered

Limestone ( O rb ito lin a b io m ic rite to O rb ito lin a b iom icrudite grading w ith pelecypod b io m ie ru d ite ); brownish gray (SYR 4 /1 ) , weathering l ig h t gray (N 7); fo s s il debris averages 0 .25 mm in s iz e ; rare chert nodules found in lower 5 fe e t (as described in u n it 3 ) ; top 0 .7 fe e t contains abundant la rge fo s s il f ra g ­ments including ribbed pelecypods and mono- p le u rid s ; she ll m ateria l commonly s i l i c i ­f ie d w ith l iv in g c a v it ie s f i l l e d w ith sparry c a lc ite ; large oyster sh e lls o f black c a lc ite - commonly p a r t ia l ly s i l i c i f i e d - also occur in upper 0 .7 fe e t ; samples C INT-I and C IN T-Ig .

Limestone (packed b io m ic r ite ) ; medium dark gray (N4) to brownish gray (SYR 4 /1 ) , weathering l ig h t gray (N 7 ); fo s s il debris averages 1 .0 mm in s ize and commonly re ­placed by sparry c a lc i te ; top 0 .5 fe e t contains numerous chert nodules (as described in u n it 3 ) ; u n it - sample C INT-J, chert nodules - sample CINT-C.

Covered

Limestone (sparse b io m ic r ite ); dark gray (N 3 ), weathering l ig h t gray (N7) to gray­ish orange (10YR 7 /4 ) ; s l ig h t hydrogen s u lf id e odor on fresh breaks; ra re fo s s il d eb ris , including m il io l id s , averages 0 .5 mm in s iz e ; u n it very poorly exposed.

Covered

Limestone (oyster b iom icrud ite grading w ith m il lo l id b io m ic r ite ); dark gray (N3) to brownish gray (SYR 4 /1 ) , weathering l ig h t gray (N 7); f a i r hydrogen s u lf id e odor on fresh breaks; abundant fo s s il debris

averages 0 .5 -1 .0 mm in s iz e ; largeoyster she lls composed o f black cal c i te -commonly p a r t ia l ly s i l i c i f i e d ; top0 .5 fo o t o f u n it lacks the oyster s h e lls , butcontains e s p e c ia lly abundant m il io lid s ;sample CINT-F.

Limestone (monopleurid b io m ic ru d ite ); dark gray (N3) to brownish gray (SYR 4 /1 ) , weathering l ig h t gray (N 7); s l ig h t hydrogen s u lf id e odor on fresh breaks, f a i r amount o f fo s s il d eb ris , including m il io l id s , averages 0 .5 -1 .0 mm in s iz e ; la rge number o f ru d is t fragments averaging 1 .0 -2 .0 cm in s ize - commonly p a r t ia l ly s i l i c i f i e d w ith sparry cal c i te f i l l i n g the l iv in g c a v it ie s ; top 0 .8 fe e t shows a gradual decrease in sh e ll f ra g ­ment s ize and lo c a lly the fragments show a crude horizon ta l s t r a t i f ic a t io n ; sample CINT-G

Limestone (sparse b io m ic rite grading to oyster b io m ic ru d ite ); dark gray (N3) to brownish gray (SYR 4 /1 ) , weathering l ig h t gray (N 7 ); s l ig h t hydrogen s u lf id e odor on fresh breaks; fo s s il d eb ris , includ ing m il io l id s , averages 0 .2 5 - 0.12 mm in s iz e ; upper portion of u n it con­ta in s coarser fo s s il debris (averages 1.0 mm in s iz e ) and la rge oyster sh e lls (as described in u n it 5 ) ; samples CINT-Hg, CINT-Hg

Limestone (packed b io m ic rite to packed b io ­m ic ru d ite ); dark gray (N3) to brownish gray (SYR 4 /1 ) , weathering l ig h t gray (N 7); abundant fo s s il d e b ris , including m il io l id s , varies in s ize through the u n it from a maximum average o f 2 .0 -3 .0 mm to a minimum average o f 0 .5 -1 .0 mm; fo s s il debris shows a crude horizon ta l s t r a t i f ic a t io n - commonly e ith e r s i l i c i f i e d or replaced by sparry c a lc i te ; lower 2 .5 fe e t contains f iv e very th in breaks th a t are recessed and very poorly exposed; they appear to be ir re g u la r zones o f crudely sorted and h o r iz o n ta lly laminated fo s s il deb ris ; upper 2 .0 fe e t contains numerous th in , ir r e g u la r , discontinuous zones o f th is crudely sorted fo s s il debris g iv ing th is portion o f the u n it a m ottled appearance; sample CINT-M.

12 Limestone (monopleurid b io m ie ru d ite ); brown­ish gray (SYR 4 /1 ) , weathering l ig h t gray (N 7); f in e fo s s il debris averaging 0 .5 -1 .0 mm; numerous la rge ru d is t fragments - commonly s i l i c i f i e d w ith sparry c a lc ite f i l l i n g the l iv in g c a v it ie s ; lo c a l ly the fo s s il f ra g ­ments are crudely h o r iz o n ta lly lam inated; sample CINT-E.

13 Limestone (packed b io m ic rite to packed b io m ie ru d ite ); medium dark gray (N4) to brownish gray (SYR 4 /1 ) , weathering l ig h t gray (N7) and dark yellow ish orange (10YR 6 /6 ) ; coarse fo s s il debris averaging 1 .0 -3 .0 mm in s iz e , w ith many fo s s il fragments s i l i c i f i e d or re ­placed by sparry c a lc i te ; lo c a l ly lenses o f lim o n ite a f te r p y r ite occur - in some instances r e l i c t cubic c ry s ta l form is preserved.

14 Limestone (b io m ic r ite ) ; dark gray (N3) to brownish gray (SYR 4 /1 ) , weathering l ig h t gray (N 7); strong hydrogen s u lf id e odor on fresh breaks; sh e ll d eb ris , including O rb ito lin a , averages 0 .3 5 -0 .1 5 mm in s iz e ; is o la te d patches o f lim o n ite pseudo- morph a f te r p y r ite c ry s ta ls occur.

15 Limestone (packed b io m ic r ite ) ; dark gray (N3) to brownish gray (SYR 4 /1 ) , weather­ing l ig h t gray (N7) and pale yellow ish orange (10YR 8 /6 ) ; s l ig h t hydrogen s u lf id e odor on fresh breaks; she ll d eb ris , inc lud ing O rb ito lin a , averages 0 .3 5 -0 .1 5 mm in s iz e ; u n it has a s l ig h t ly m ottled appearance.

Covered

16 Limestone (packed b io m ic r ite ); dark gray(N3) to brownish gray (SYR 4 /1 ) , weathering l ig h t gray (N7) and pale yellow ish orange (10YR 8 /6 ) ; s l ig h t hydrogen s u lf id e odor on fresh breaks; fo s s il debris averages 0 .2 5 - 0 .50 mm in s iz e ; lower 1 fo o t has a s l ig h t ly m ottled appearance; upper 0 .6 fo o t has small local lenses (burrow f i l l s ) r ic h in O rb ito lin a - due to mixing w ith sediment from overly ing u n it ; sample C INT-0.

84

2 .7 86 .0

0 .5 86.5

1.6 88.1

1 .0 89.1

1 .0 90.1

1 .6 91.7

85

Covered

17 Limestone (O rb ito lin a b io m ie ru d ite ) ; medium dark gray (N4) to dark gray (N 3 ), weathering l ig h t gray (N7) and grayish orange (10YR 7 /4 ) ; s l ig h t hydrogen s u lf id e odor on fresh breaks; u n it has a m ottled appearance; u n it recessed forming overhang w ith massive u n it above; sample CINT-N.

18 Limestone (o yster b iom icrud ite gradingto b io m ic rite and pelecypod b io m ic ru d ite ); l ig h t brownish gray (5YR 6 /1 ) , weathering l ig h t gray (N 7); the lower 6 .5 fe e t con­ta in s numerous la rge oyster she lls o f black cal c ite - commonly p a r t ia l ly s i l i c i f i e d , sometimes in the form o f euhedral quartz c ry s ta ls ; the s h e lls show a crude ho rizo n ta l s t r a t i f ic a t io n , and general concave-up o r ie n ta tio n ; above is a 4 inch zone devoid o f large oyster sh e lls and th is in turn grades in to a 1 fo o t zone dominated by 1 .0 -2 .0 cm pelecypod she ll fragments commonly s i l i c i ­f ie d by sparry c a lc i te ; sample CINT-P.

19 Limestone (b io m ic r ite grading to O rb ito lin a b io m ic ru d ite ); brownish gray (SYR 4 /1 ) to dark gray (N 3 ), weathering l ig h t gray (N 7); s lig h t hydrogen s u lf id e odor on fresh breaks; fo s s il d eb ris , including m il io l id s , averages 0 .5 0 - 0.25 mm in s ize and is commonly replaced by sparry c a lc i te ; u n it has a m ottled appearance; top 4 inches is r ic h in O rb ito lin a .

Covered

20 Limestone (b io m ic r ite ) ; same as u n it 19.

Covered

21 Limestone (b io m ic r ite ) ; same as u n it 19; m o ttlin g is e s p e c ia lly w ell developed in th is u n it ; m ottled appearance seems to be due to small ir re g u la r zones w ith a crude sortin g w ith in the bed; O rb ito lin a is commonly concentrated in these crudely sorted zones.

1 .4 93.1

2 .0 95.1

7 .5 102.6

1 .3 103.9

0 .5 104.4

2 .3 106.7

1.1 107.8

2 .4 110.2

Covered 5 .0 115.2

86

22 Limestone (packed b io m ic r ite ) ; dark gray (N3) 8 .3 123.5to brownish gray (SYR 4 /1 ) , weathering l ig h tgray (N 7); f a i r hydrogen s u lf id e odor on fresh breaks; abundant fo s s il d eb ris , including m ilio lid s and rare O rb ito lin a , averages 0 .5 0 -0 .2 5 mm in s iz e ; a few large fragments of h igh -sp ired gastropods and oyster sh e lls present; the u n it shows varying degrees o f m o ttlin g , s im ila r to th a t described in u n it 11; there are sm all, ir re g u la r , discontinuous zones o f crudely sorted fo s s il debris w ith in more poorly sorted zones, ranging to d is c re te , la t e r a l ly continuous beds up to 0 .8 fo o t th ic k interbedded w ith the more poorly sorted beds; these la t e r a l ly continuous crudely sorted beds weather e a s ily and are recessed along the c l i f f face; sample CINT-R.

23 Limestone (oyster b iom icrud ite grading w ith 12 .0 135.5packed b io m ic r ite ); dark gray (N3) to brownishgray (SYR 4 /1 ) , weathering l ig h t gray (N 7); s lig h t hydrogen s u lf id e odor on fresh breaks; u n it is d iv ided in to 3 sections; middle section reaches a maximum thickness o f 1 .0 fo o t , but pinches out la t e r a l ly ; i t is poorly exposed and commonly recessed along the c l i f f face ; i t is analogous to the recessed beds described in u n it 22; the upper and lower sections are q u ite s im ila r ; they are dominated by la rg e oyster shell fragm ents, though ru d is t (c a p rin id and monopleurid) sh e ll fragments are abundant a lso ; there is abundant f in e fo s s il d eb ris , including m il io l id s ; small patches o f lim o n ite pseudomorph a f te r p y r ite c ry s ta ls occur, e s p e c ia lly in the upper sec tion ; s i l i c i f i c a t io n o f fo s s il m a te r ia l, sometimes in the form o f euhedral quartz c ry s ta ls , is common; the la rge oyster fragments show a crude horizon ta l s t r a t i f ic a t io n , and general concave-up o r ie n ta tio n , being best developed in the upper sec tion ; the lower section la t e r a l ly becomes m o ttled , w ith la rge fo s s il fragments becoming q u ite ra re - i t becomes s im ila r to the middle sec tion ; sample CINT-S.

24 Limestone (packed b io m ic r ite ); dark gray (N3) 2 .5 137.0to brownish gray (SYR 4 /1 ) , weathering l ig h tgray (N 7); s l ig h t hydrogen s u lf id e odor on

fresh breaks; abundant fo s s il d eb ris , inc lud ing m ilio lid s and O rb ito lin a , averages 0 .5 0 - 0.25 ran in s iz e ; upper 1 .5 fe e t appears s l ig h t ly m ottled - apparently due to s l ig h t v a ria tio n s in so rtin g (as described p re v io u s ly ).

END OF SECTION - section ends a t top o f southwest­facing c l i f f s .

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